Fixes #6549 - Add :tests_to_skip to plugin registration block

This PR allows a plugin to do something like
```
Foreman::Plugin.register :foreman_discovery do                                 
  tests_to_skip "EncryptableTest" => ["this string IS encryptable and NOT decryptable"],
                "HostTest" => ["should not save without a hostname"]
end
```
i.e. a map of test types to arrays of test names to skip

~~This is detected where `test` (which is aliased against `it` later in the test_helper) is defined - we jump in and monkeypatch MiniTest to check if this is a prohibited test class+name. If it is, we just define the test as a skip.~~
This alters the array of tests to run by using a custom test runner. We step into the `_run_suite` method and add an extra round of filtering against `Foreman::Plugin.tests_to_skip`

This is slight hacky - the downside is:
1) ~~monkey_patching MiniTest~~ replacing `_run_suite` and not calling `super`
2) ~~it only works for the newer `test "foo" ... end` style tests, `def test_foo ... end` can't be caught this way~~   fixed.

Feedback welcome - as I'm out of ideas as to how to do it better :)