[8.x] Fixes attempt to log deprecations on mocks

[nunomaduro]

This pull request fixes an issue that may happen when testing code looks like so:

    public function test_example()
    {
        Log::spy();

        // Error: Call to a member function warning() on null
        str_contains(null, null);
    }

In other words, the framework, and the attempt to log the deprecation, tries to resolve the Log instance that is a mock in this case. Now, because in tests, we only want to raise deprecations as errors when using the withoutDeprecationHandling, we can safely disable deprecation logs on unit tests.

This needs to be included on L9 too.

Fixes #40942 (comment), and Mior's issue on Nova this morning: