Changeset 51462

Tests: Replace assertContains() with assertStringContainsString() when used with strings.

Using the assertContains() and assertNotContains() methods with string haystacks was deprecated in PHPUnit 8 and removed in PHPUnit 9.

While WordPress test suite currently only supports PHPUnit up to 7.5.x, this allows us to switch to newer assertions ahead of adding full support for PHPUnit 8+.

These methods introduced in PHPUnit 7.5 should be used as an alternative:

• assertStringContainsString()
• assertStringContainsStringIgnoringCase
• assertStringNotContainsString()
• assertStringNotContainsStringIgnoringCase

As WordPress currently uses PHPUnit 5.7.x to run tests on PHP 5.6, polyfills for these methods were added to the WP_UnitTestCase class for PHPUnit < 7.5.

Follow-up to [51331], [51451], [51461].

Props jrf, dd32, SergeyBiryukov.
See #53363, #46149.