Currently there is a set of debug configurations maintained in the mediawiki/conf.d directory of the integration/jenkins.git repository. This is included in (most) Jenkins jobs that run MediaWiki tests (PHPUnit, QUnit or otherwise).

https://github.com/wikimedia/integration-jenkins/tree/master/mediawiki/conf.d

When working locally (e.g. via MediaWiki-Vagrant) or when running tests via Travis CI, however, these settings don't apply.

There are some Jenkins and/or Wikimedia specific things here, but for the most part it might make sense to maintain these in MediaWiki core instead (e.g. tests/DebugTestSettings.php or some such).

At the very least this would contain the following:

// Debugging: PHP
error_reporting( -1 );
ini_set( 'display_errors', 1 );
// Debugging: MediaWiki
$wgDevelopmentWarnings = true;
$wgShowDBErrorBacktrace = true;
$wgShowExceptionDetails = true;
$wgShowSQLErrors = true;

Context: Debugging Postgres failures on Travis CI (T75174) is difficult given by default $wgShowExceptionDetails is false and thus a useless error was shown until manually a pull-request was drafted on GitHub to re-run the tests with this enabled.

We could append this within .travis.yml only, but given that we want the same in Jenkins and other places were we run tests, it might make more sense if MediaWiki had a way of enabling these (and other settings) by default when running tests. Or at least provided a file that can be easily included from various places, which could happen by default in phpunit.php, or perhaps via a command-line switch (opt-in).