Use default FileAlg to implement MockFileAlg, take 2

[fthomas]

This changes MockFileAlg such that it not only modifies the MockState but also files in the filesystem via the default FileAlg implementation. The main motivation for this change is to make using actual Git commands in tests possible. Another advantage is that we increase the coverage of the default FileAlg. A disadvantage is that tests can now be influenced by existing or deleted files. So we need to be more careful now that tests do not write files in the same location.

[codecov]

Codecov Report

Merging #1962 (79d97e8) into master (cc9750b) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1962      +/-   ##
==========================================
+ Coverage   78.35%   78.40%   +0.04%     
==========================================
  Files         129      129              
  Lines        2227     2227              
  Branches       60       60              
==========================================
+ Hits         1745     1746       +1     
+ Misses        482      481       -1     

Impacted Files	Coverage Δ
.../main/scala/org/scalasteward/core/io/FileAlg.scala	96.87% <0.00%> (+3.12%)	⬆️

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