feat: allow using groups layout with cpp tester

[kalabukdima]

C++ tester only supports flat layout right now because it runs build as cmake ${task_dir}/.. assuming that the project root is located one directory up from the task dir.
This PR adds support for the groups layout while preserving functionality for the flat layout.

[codecov]

Codecov Report

Merging #30 (c5e7375) into main (6389346) will increase coverage by 8.31%.
Report is 11 commits behind head on main.
The diff coverage is 64.07%.

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   36.94%   45.25%   +8.31%     
==========================================
  Files          24       25       +1     
  Lines        1903     2022     +119     
==========================================
+ Hits          703      915     +212     
+ Misses       1200     1107      -93     

Flag	Coverage Δ
unittests	45.25% <64.07%> (+8.31%)	⬆️

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Files Changed	Coverage Δ
checker/__main__.py	0.00% <0.00%> (ø)
checker/actions/check.py	0.00% <0.00%> (ø)
checker/actions/contributing.py	0.00% <ø> (ø)
checker/actions/export.py	0.00% <0.00%> (ø)
checker/actions/grade.py	0.00% <0.00%> (ø)
checker/actions/grade_mr.py	0.00% <0.00%> (ø)
checker/executors/sandbox.py	87.77% <ø> (ø)
checker/utils/git.py	17.50% <ø> (ø)
checker/utils/manytask.py	80.85% <ø> (+62.48%)	⬆️
checker/testers/make.py	44.44% <20.00%> (+44.44%)	⬆️
... and 13 more