Fix unreachable assertion in Provenance unit test

[HealthyPear]

The assertion was inside a fixture after a return statement, so it was never called.

I created a new unit-test that uses the fixture and calls that assertion.

[codecov]

Codecov Report

Merging #2072 (4bff97c) into master (428d114) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 4bff97c differs from pull request most recent head abf4e62. Consider uploading reports for the commit abf4e62 to get more accurate results

@@           Coverage Diff           @@
##           master    #2072   +/-   ##
=======================================
  Coverage   92.43%   92.43%           
=======================================
  Files         197      197           
  Lines       16183    16185    +2     
=======================================
+ Hits        14958    14960    +2     
  Misses       1225     1225           

Impacted Files	Coverage Δ
ctapipe/core/tests/test_provenance.py	100.00% <100.00%> (ø)

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