Add tests for nf-core subworkflows create-test-yml

[Aratz]

This PR adds unit-tests for nf-core subworkflows create-test-yml.

The code in nf_core/subworkflows/test_yml_builder.py is very similar to its module equivalent, so the new tests are more or less a copy of the module tests. Maybe it would be worth refactoring SubworkflowTestYmlBuilder and ModuleTestYmlBuilder to avoid code duplication?

Closes #1895

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!

[codecov]

Codecov Report

Merging #2219 (8d81588) into dev (f8ef685) will increase coverage by 0.96%.
The diff coverage is 33.33%.

❗ Current head 8d81588 differs from pull request most recent head 20f82c0. Consider uploading reports for the commit 20f82c0 to get more accurate results

@@            Coverage Diff             @@
##              dev    #2219      +/-   ##
==========================================
+ Coverage   71.99%   72.96%   +0.96%     
==========================================
  Files          78       78              
  Lines        8385     8385              
==========================================
+ Hits         6037     6118      +81     
+ Misses       2348     2267      -81     

Impacted Files	Coverage Δ
nf_core/subworkflows/test_yml_builder.py	49.37% <33.33%> (+33.89%)	⬆️

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[Aratz]

@nf-core-bot fix linting

[mashehu]

Looks good! Thanks for doing this!

[Aratz]

Thanks for the reviews!