[bugfix] Fix --restore-session failure when ReFrame root directory is not in sys.path[0]

[rsarm]

Close #2135

[rsarm]

@mahendrapaipuri could you have a look?

[codecov-commenter]

Codecov Report

Merging #2201 (6f7c4ca) into master (9804e1c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2201   +/-   ##
=======================================
  Coverage   85.54%   85.54%           
=======================================
  Files          55       55           
  Lines        9745     9746    +1     
=======================================
+ Hits         8336     8337    +1     
  Misses       1409     1409           

Impacted Files	Coverage Δ
reframe/utility/__init__.py	92.76% <100.00%> (+0.01%)	⬆️

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[mahendrapaipuri]

@rsarm Looks good. Just a minor suggestion. Do you think moving

rfm_root = os.path.realpath(os.path.join(os.path.dirname(__file__), '../..'))

into a new function like get_rfm_root makes sense? This makes it more modular and reusable if we ever need ReFrame root directory in further developments.

[rsarm]

What about replacing by here reframe.INSTALL_PREFIX too?