Tests: Using assertContains() with string haystacks is deprecated

[gesinn-it-gea]

Using assertContains() with string haystacks is deprecated and will not be supported in PHPUnit 9. Refactor your test to use assertStringContainsString() or assertStringContainsStringIgnoringCase() instead.

See CI output for examples.

[malberts]

There's a trait you can add. I did this for SRF:
SemanticMediaWiki/SemanticResultFormats@054e482

[gesinn-it-gea]

@malberts thanks for the hint! This issue was more meant as a heads-up for future work...