Fix Crash in Maven builds, fixes #200

[dmfs]

In some builds (e.g. maven) the test class path may be shorter than four elements. This now no longer causes a crash.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.26%. Comparing base (ba59578) to head (4b003ec).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...testengine/testdescriptor/ClassTestDescriptor.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #201      +/-   ##
============================================
- Coverage     96.31%   96.26%   -0.05%     
- Complexity      883      884       +1     
============================================
  Files           230      230              
  Lines          1980     1981       +1     
  Branches         64       64              
============================================
  Hits           1907     1907              
  Misses           63       63              
- Partials         10       11       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.