[testlib] Make GPU burn test a library test

[vkarak]

More particularly, this PR offers the following:

• Converts the GPU burn test into a library test.
• Bases the the CSCS test on this new library test.
• Introduces new command line arguments to the GPU burn test itself, so as to control better its behavior.
• Introduces a new source file organisation for the GPU microbenchmarks (currently, only the GPU burn test follows the new structure, but the rest of the tests will have to follow it as soon as they are updated).
• The CSCS config is updated with metadata on its partitions and environments.
• A new convenience method is added to the SystemPartition that allows to select the devices of a certain type.

[pep8speaks]

Hello @vkarak, Thank you for submitting the Pull Request!

• In the file hpctestlib/microbenchmarks/gpu/gpu_burn.py:

Line 104:80: E501 line too long (83 > 79 characters)

Do see the ReFrame Coding Style Guide

[codecov-commenter]

Codecov Report

Merging #2503 (d4874bf) into master (cec5eee) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #2503      +/-   ##
==========================================
- Coverage   85.87%   85.86%   -0.01%     
==========================================
  Files          58       58              
  Lines       10775    10777       +2     
==========================================
+ Hits         9253     9254       +1     
- Misses       1522     1523       +1     

Impacted Files	Coverage Δ
reframe/core/systems.py	89.49% <50.00%> (-0.34%)	⬇️

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[codecov-commenter]

Codecov Report

Merging #2503 (e08af94) into master (cec5eee) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #2503      +/-   ##
==========================================
- Coverage   85.87%   85.86%   -0.01%     
==========================================
  Files          58       58              
  Lines       10775    10777       +2     
==========================================
+ Hits         9253     9254       +1     
- Misses       1522     1523       +1     

Impacted Files	Coverage Δ
reframe/core/systems.py	89.49% <50.00%> (-0.34%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cec5eee...e08af94. Read the comment docs.