Add CI testing for GitLab

[ErikDanielsson]

Added both pytests and command line tests of the --git-remote option with the nf-core/modules-test repository. Will close #1646.

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 #1665 (e2c11b5) into dev (cbe5b2a) will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev    #1665      +/-   ##
==========================================
+ Coverage   64.43%   64.58%   +0.15%     
==========================================
  Files          56       56              
  Lines        6574     6574              
==========================================
+ Hits         4236     4246      +10     
+ Misses       2338     2328      -10     

Impacted Files	Coverage Δ
nf_core/modules/install.py	70.51% <0.00%> (+1.28%)	⬆️
nf_core/modules/modules_json.py	32.15% <0.00%> (+1.41%)	⬆️
nf_core/modules/modules_repo.py	80.26% <0.00%> (+3.28%)	⬆️

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[mirpedrol]

LGTM :)

[ErikDanielsson]

Thanks (: