[9.x] Fixes memory leak on TestCase handlers

[nunomaduro]

This pull request addresses one of the memory leaks being mentioned at #44214, by performing a proper cleaning of the $originalExceptionHandler and $originalDeprecationHandler on the Laravel's base test case class.

On Vapor's test suite, after having #44306 (33% less memory), and after disabling bugsnag (14% less memory) - here is the results on 600 tests: 1% less memory used.

Before…
Time: 01:02.732, Memory: 168.50 MB

After…
Time: 01:02.595, Memory: 166.50 MB