[NFC] Resolve phpunit warning about using assertContains on strings

[seamuslee001]

Overview

This resolves the following warning in phpunit runs

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

Before

Warning is shown in phpunit runs

After

No warning

ping @eileenmcnaughton @colemanw @totten @demeritcowboy

[civibot]

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[totten]

+1. If the test passes, it's good.