expectExceptionMessage() uses str_contains() to verify the exception message. This means that expectExceptionMessage('foo') also passes when the actual exception message is 'foobar'. There is currently no way to assert that the exception message is exactly a given string.

Introduce expectExceptionMessageIs() that uses === instead of str_contains() for exception message verification.

Additionally, introduce expectExceptionMessageIsOrContains() as an explicitly-named replacement for expectExceptionMessage() that preserves the current str_contains() behavior.

Motivated by #6425.