tests: Use assertStringContainsString for string contains, to support PHPUnit 9
tests: Use assertStringContainsString for string contains, to support PHPUnit 9

Change-Id: I2fac31b97577357d49152e529e99f84972ed7935

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui