Add test coverage for stub file ignore

[steverice]

This adds some simple test coverage for 7d32365.

[codecov]

Codecov Report

Merging #145 (76de0ae) into main (d97a8af) will decrease coverage by 0.4%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #145     +/-   ##
=======================================
- Coverage   98.7%   98.3%   -0.5%     
=======================================
  Files          3       3             
  Lines        472     474      +2     
=======================================
  Hits         466     466             
- Misses         6       8      +2     

Impacted Files	Coverage Δ
flake8_type_checking/checker.py	98.0% <100.0%> (-0.5%)	⬇️
flake8_type_checking/plugin.py	100.0% <100.0%> (ø)