[FindNextAction] Introduce unit tests

[Wittmaxi]

Introduces unit tests for the FindNextAction.
I don't achieve 100% test coverage, but I test for the most important parts. Appart from the "naive" functionality, in particular I test for:

1. search options are taken into account
2. the search history is honored
3. whole - word search is only performed if the selection is a word
4. RegExes are correctly escaped

[github-actions]

Test Results

 1 802 files   -  19   1 802 suites   - 19   2h 2m 3s ⏱️ + 10m 33s
 7 724 tests + 10   7 493 ✅ +  7  228 💤 ±0  2 ❌ +2  1 🔥 +1 
23 974 runs   - 329  23 230 ✅  - 326  741 💤  - 6  2 ❌ +2  1 🔥 +1 

For more details on these failures and errors, see this check.

Results for commit 1d02099. ± Comparison against base commit bbd9101.

♻️ This comment has been updated with latest results.

[Wittmaxi]

The test is now in the editors.test package. Reason is that I need to start a whole editor in order to test the FindNextAction using IDE.openEditor. Depending on IDE in the workbench.editors package is problematic and such @HeikoKlare and I convened on moving the test to a more sensible place.

[Wittmaxi]

@HeikoKlare thank you for updating the PR. I had copied over the method for opening an editor from another test - yet I agree that failing siletnyl is not a good idea.

[HeikoKlare]

Thank you for adding the tests, @Wittmaxi! They are executed and succeed as expected.
Other failing tests are unrelated and documented: #751, #370