[feat] Enable easy testing of non-default Cray programming environments

[vkarak]

This PR adds the option --non-default-craype that enables ReFrame to test non-default Cray programming environments. This option causes ReFrame to export properly the LD_LIBRARY_PATH after loading any additional user modules. Use as follows:

module load cdt/19.09   # load target PE
reframe ... --non-default-craype -r

Fixes #895.

[codecov-io]

Codecov Report

Merging #949 into master will decrease coverage by 0.01%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master     #949      +/-   ##
==========================================
- Coverage   91.64%   91.63%   -0.02%     
==========================================
  Files          79       79              
  Lines       10482    10492      +10     
==========================================
+ Hits         9606     9614       +8     
- Misses        876      878       +2

Impacted Files	Coverage Δ
reframe/core/runtime.py	88.35% <100%> (+0.24%)	⬆️
reframe/frontend/cli.py	80.32% <66.66%> (+0.06%)	⬆️
reframe/core/pipeline.py	93.77% <91.66%> (-0.14%)	⬇️
reframe/frontend/dependency.py	95.91% <0%> (-1.03%)	⬇️

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[victorusu]

lgtm. I just have a question regarding the API growth. I wonder if we want to make use of kwargs.
What was not so clear to me also, is the use case.
If the system is a non-default cray but contains a Cray PE, then this should be reported in the definition of the programming environment, no? Or we just want a flag to trigger it?