Fix --key flag for nf-core lint for module lint tests

[ErikDanielsson]

When running nf-core module --key <module-test> all pipeline lint tests were run in addition to the supplied module test (#1681). With this PR only the tests specified by the --key option are run

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 #1682 (27d08b4) into dev (cc1ab61) will decrease coverage by 0.06%.
The diff coverage is 36.36%.

@@            Coverage Diff             @@
##              dev    #1682      +/-   ##
==========================================
- Coverage   67.99%   67.93%   -0.07%     
==========================================
  Files          56       56              
  Lines        6609     6614       +5     
==========================================
- Hits         4494     4493       -1     
- Misses       2115     2121       +6     

Impacted Files	Coverage Δ
nf_core/lint/__init__.py	73.66% <36.36%> (-1.97%)	⬇️

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[mirpedrol]

LGTM :)

[ErikDanielsson]

Thanks for the review!