fix missing output files in test.yaml

[mirpedrol]

Closes #1564

When output files are inside a folder (e.g. index files from bowtie2/align module), os.walk() needs the parameter followlinks=True when creating the test.yml file to be able to retrieve all files.

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!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #1570 (e9be065) into dev (8f5989d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev    #1570   +/-   ##
=======================================
  Coverage   64.72%   64.72%           
=======================================
  Files          54       54           
  Lines        6246     6246           
=======================================
  Hits         4043     4043           
  Misses       2203     2203           

Impacted Files	Coverage Δ
nf_core/modules/test_yml_builder.py	48.40% <100.00%> (ø)

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[ewels]

Specifically if using softlinks, right? Can't see the source that triggered the error, but LGTM! 👍🏻