Add stubs to create test yml

[nvnieuwk]

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 #2476 (efe344a) into dev (66dbf0b) will decrease coverage by 0.08%.
Report is 2 commits behind head on dev.
The diff coverage is 51.61%.

@@            Coverage Diff             @@
##              dev    #2476      +/-   ##
==========================================
- Coverage   70.63%   70.56%   -0.08%     
==========================================
  Files          87       87              
  Lines        9460     9471      +11     
==========================================
+ Hits         6682     6683       +1     
- Misses       2778     2788      +10     

Files	Coverage Δ
nf_core/modules/test_yml_builder.py	49.77% <50.00%> (-1.14%)	⬇️
nf_core/subworkflows/test_yml_builder.py	48.16% <52.94%> (-0.80%)	⬇️

[mirpedrol]

LGTM!
Just a small comment, what do you think about making the stub test the last one instead of the first?