Refactor test code so it can be excluded from code coverage

[scouten-adobe]

Changes in this pull request

Test code is not part of the library as consumed by third parties, so (IMHO) should not be included in coverage reports.

Since it's not (yet?) possible to exclude code at function or block level in stable builds, using this approach so that test code can be excluded via cargo-llvm-cov's exclude by file path option.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[scouten-adobe]

Alternative to #65.

[codecov]

Codecov Report

Merging #66 (118e9d8) into main (aef54aa) will decrease coverage by 11.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main      #66       +/-   ##
===========================================
- Coverage   84.09%   73.07%   -11.02%     
===========================================
  Files           4        4               
  Lines         308      182      -126     
===========================================
- Hits          259      133      -126     
  Misses         49       49               

Impacted Files	Coverage Δ
src/lib.rs	100.00% <ø> (ø)
src/xmp_date_time.rs	83.33% <ø> (-5.13%)	⬇️
src/xmp_file.rs	53.76% <ø> (-21.67%)	⬇️
src/xmp_meta.rs	95.71% <ø> (-1.46%)	⬇️

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[limitedAtonement]

Since it's not (yet?) possible to exclude code at function or block level...

Is this still the case? Is there a way to exclude code coverage reports at a function, block, or statement level?

[taiki-e]

The --ignore-filename-regex is not needed and the tests directory should be excluded by default wherever it is located.
https://github.com/taiki-e/cargo-llvm-cov/blob/3f21ce8c7e21b38bf85112757cd119af8360af76/src/main.rs#L1227