Replace assertContains() on strings with assertStringContainsString() or assertStringContainsStringIgnoringCase()

Problem/Motivation

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

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet