Modernize Text JUnit tests

[akurtakov]

Use assertThrows and remove dummy assertion messages as they give no valuable information.

[github-actions]

Test Results

   545 files  + 6     545 suites  +6   33m 19s ⏱️ + 2m 36s
 4 367 tests +37   4 351 ✅ +35   16 💤 +3  0 ❌  - 1 
16 616 runs  +37  16 502 ✅ +35  114 💤 +3  0 ❌  - 1 

Results for commit d60d8e4. ± Comparison against base commit 5a22b50.

♻️ This comment has been updated with latest results.

[akurtakov]

@HeikoKlare Do you see local failure in test_backspaceAndDelete on gtk3 on wayland ?

[HeikoKlare]

I have executed that method in both the StyledText and Text test classes. They both succeed when excuted standalone and also when executed via the AllNonBrowserTests suite.
However, I currently don't have a native Linux system available and just executed it on WSL, so not that meaningful (though it's running GTK3 on Wayland).