Linting checks test files for TODO statements as well as the main module code

[ewels]

Closes #1271

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 #1443 (bff0b14) into dev (ae06289) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1443      +/-   ##
==========================================
+ Coverage   65.67%   65.72%   +0.05%     
==========================================
  Files          50       50              
  Lines        5844     5853       +9     
==========================================
+ Hits         3838     3847       +9     
  Misses       2006     2006              

Impacted Files	Coverage Δ
nf_core/lint/pipeline_todos.py	94.11% <100.00%> (+0.56%)	⬆️
nf_core/modules/lint/module_todos.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ae06289...bff0b14. Read the comment docs.