fix(crashmanager): prevent duplicate testcase extensions

[pyoor]

Store template basenames, migrate existing filenames, and preserve the original testcase extension when filing bugs or comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.94%. Comparing base (9886919) to head (69a4314).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1175      +/-   ##
==========================================
+ Coverage   63.45%   66.94%   +3.48%     
==========================================
  Files         181      182       +1     
  Lines       12287    12288       +1     
  Branches      881      873       -8     
==========================================
+ Hits         7797     8226     +429     
+ Misses       4456     4028     -428     
  Partials       34       34              

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