MediaWiki's PhpUnit wrapper has a --reuse-db, which is intended to be used together with the --use-normal-tables option. It instructs the MediaWikiIntegrationTest base class to try and re-use any existing tables with the phpunit_ prefix, and to also keep any tables with the phpunit_ prefix when exiting. Normally, such tables are deleted when unit tests are complete.

This feature has been broken since at least MW 1.32: when --reuse-db is specified, the step that would create these tables is skipped entirely, instead of just omitting any existing tables. This means that that attempting to run tests with --reuse-db set will fail immediately, since all test tables are missing.

We could fix this, but since it has been broken for three years, and apparently nobody has complained about it, the feature should just be removed. Doing so will make it easier to implement T261848.

The intended use case isn't clear either. Perhaps the idea was to manually inspect the state of the database after the test run, or to manually create a database state to run against? But since tables get truncated between tests based on MediaWikiIntegrationTest's tablesUsed field, that state will be incomplete at best...