Add a replacement for `assertInternalType`
Add a replacement for `assertInternalType`

This was removed in PHPUnit 9, but its replacements aren't available in
PHPUnit 4 (which we still support)

We have to do this with a new Assert class, rather than implementing it
in our TestCase class because otherwise we would need to exactly match
PHPUnit's method signature. This isn't possible because PHPUnit added
parameter and return types to their method, which we can't replicate
because of PHP 5 compatibility

bugsnag/bugsnag-php,bugsnag/bugsnag-php,bugsnag/bugsnag-php