[feat] Add support for running the unit tests offline

[vkarak]

This essentially introduces a new option, --rfm-offline to the test_reframe.py script that will skip all the unit tests that need Internet access.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.38%. Comparing base (acb62bc) to head (64a84f6).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3626      +/-   ##
===========================================
- Coverage    91.42%   91.38%   -0.04%     
===========================================
  Files           62       62              
  Lines        13520    13520              
===========================================
- Hits         12360    12355       -5     
- Misses        1160     1165       +5     

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