PHPUnit fails on TravisCI, not sure how to debug - wordpress

Question

I'm still trying to teach myself Travis / CI so apologies if the answer is obvious.
I'm trying to determine why my Travis build for a project I've inherited is failing for PHP 7.1 but not 7.2.
Here is the .travis.yml config:
sudo: false
language: php
# Cache some data across builds for performance.
cache:
apt: true
directories:
- node_modules
- vendor
- $HOME/.composer/cache
notifications:
email:
on_success: never
on_failure: change
branches:
only:
- master
matrix:
include:
- php: 7.2
env: WP_Version=nightly
- php: 7.1
env: WP_VERSION=latest WP_TRAVISCI=phpcs
before_script:
- export PATH="$HOME/.composer/vendor/bin:$PATH"
- phpenv config-rm xdebug.ini
- |
if [[ ! -z "$WP_VERSION" ]] ; then
bash bin/install-wp-tests.sh wordpress_test root '' localhost $WP_VERSION
composer global require "phpunit/phpunit=5.7.*"
fi
- |
if [[ "$WP_TRAVISCI" == "phpcs" ]] ; then
composer global require wp-coding-standards/wpcs
phpcs --config-set installed_paths $HOME/.composer/vendor/wp-coding-standards/wpcs
fi
- mysql --version
- phpenv versions
- php --version
script:
- |
if [[ ! -z "$WP_VERSION" ]] ; then
phpunit --version
phpunit
WP_MULTISITE=1 phpunit
fi
- |
if [[ "$WP_TRAVISCI" == "phpcs" ]] ; then
phpcs --standard=phpcs.ruleset.xml $(find . -name '*.php')
fi
# If the plugin has any JavaScript or CSS that should be processed,
# add the below to their respective sections so that Grunt is installed
# and all default tasks are run.
#
# To the matrix:
#
# - php: 7.0
# env: WP_TRAVISCI=grunt
#
# To before_script:
#
# - npm --version
# - node --version
# - nvm install stable
# - npm install -g grunt-cli
# - npm install
# - npm prune
# - npm --version
# - node --version
#
# To script:
#
# - |
# if [[ "$WP_TRAVISCI" == "grunt" ]] ; then
# grunt --version
# grunt default
# fi
Here is the Travis log:
Setting environment variables from .travis.yml
$ export WP_VERSION=latest
$ export WP_TRAVISCI=phpcs
0.02s$ phpenv global 7.1 2>/dev/null
pearrc
Writing /home/travis/.pearrc
0.01s$ phpenv global 7.1
0.87s$ composer self-update
Updating to version 1.9.1 (stable channel).
Downloading (100%)
Use composer self-update --rollback to return to version 1.8.4
cache.1
Setting up build cache
$ php --version
PHP 7.1.27 (cli) (built: Mar 13 2019 07:54:47) ( ZTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2018 Zend Technologies
with Zend OPcache v7.1.27, Copyright (c) 1999-2018, by Zend Technologies
with Xdebug v2.7.0, Copyright (c) 2002-2019, by Derick Rethans
$ composer --version
Composer version 1.9.1 2019-11-01 17:20:17
before_script.1
0.00s$ export PATH="$HOME/.composer/vendor/bin:$PATH"
before_script.2
0.02s$ phpenv config-rm xdebug.ini
before_script.3
14.36s$ if [[ ! -z "$WP_VERSION" ]] ; then
before_script.4
2.96s$ if [[ "$WP_TRAVISCI" == "phpcs" ]] ; then
before_script.5
0.02s$ mysql --version
before_script.6
0.05s$ phpenv versions
before_script.7
0.06s$ php --version
0.28s$ if [[ ! -z "$WP_VERSION" ]] ; then
phpunit --version
phpunit
WP_MULTISITE=1 phpunit
fi
PHPUnit 5.7.27 by Sebastian Bergmann and contributors.
PHP Warning: mysqli_real_connect(): (HY000/2002): No such file or directory in /tmp/wordpress/wp-includes/wp-db.php on line 1633
wp_die called
Message : <p><code>No such file or directory</code></p>
<h1>Error establishing a database connection</h1>
<p>This either means that the username and password information in your <code>wp-config.php</code> file is incorrect or we can’t contact the database server at <code>localhost</code>. This could mean your host’s database server is down.</p>
<ul>
<li>Are you sure you have the correct username and password?</li>
<li>Are you sure you have typed the correct hostname?</li>
<li>Are you sure the database server is running?</li>
</ul>
<p>If you’re unsure what these terms mean you should probably contact your host. If you still need help you can always visit the WordPress Support Forums.</p>
Title :
PHP Warning: mysqli_real_connect(): (HY000/2002): No such file or directory in /tmp/wordpress/wp-includes/wp-db.php on line 1633
wp_die called
Message : <p><code>No such file or directory</code></p>
<h1>Error establishing a database connection</h1>
<p>This either means that the username and password information in your <code>wp-config.php</code> file is incorrect or we can’t contact the database server at <code>localhost</code>. This could mean your host’s database server is down.</p>
<ul>
<li>Are you sure you have the correct username and password?</li>
<li>Are you sure you have typed the correct hostname?</li>
<li>Are you sure the database server is running?</li>
</ul>
<p>If you’re unsure what these terms mean you should probably contact your host. If you still need help you can always visit the WordPress Support Forums.</p>
Title :
The command "if [[ ! -z "$WP_VERSION" ]] ; then
phpunit --version
phpunit
WP_MULTISITE=1 phpunit
fi
" exited with 1.
0.70s$ if [[ "$WP_TRAVISCI" == "phpcs" ]] ; then
phpcs --standard=phpcs.ruleset.xml $(find . -name '*.php')
fi
The command "if [[ "$WP_TRAVISCI" == "phpcs" ]] ; then
phpcs --standard=phpcs.ruleset.xml $(find . -name '*.php')
fi
" exited with 0.
cache.2
store build cache
Done. Your build exited with 1.
I can see the error message is related to the database, but I'm not sure where to look to debug it.
The PHP7.2 job passes.