[feat] Update retries stage/output directory naming convention

[omlins]

Modifies the behaviour of ReFrame to create stage and output folders as in the following example:

/path/to/stage/sys/part/env/testname
/path/to/stage/sys/part/env/testname_retry1
/path/to/stage/sys/part/env/testname_retry2

Closes #536.

[vkarak]

@jenkins-cscs retry daint

[teojgo]

Tested it and it works fine.

[vkarak]

@omlins I fixed the PR myself. Please have a look at the changes.

[codecov-io]

Codecov Report

Merging #580 into master will decrease coverage by 0.03%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #580      +/-   ##
==========================================
- Coverage   91.59%   91.55%   -0.04%     
==========================================
  Files          72       72              
  Lines        8954     8962       +8     
==========================================
+ Hits         8201     8205       +4     
- Misses        753      757       +4

Impacted Files	Coverage Δ
reframe/core/pipeline.py	91.72% <100%> (+0.02%)	⬆️
reframe/core/runtime.py	87.05% <86.66%> (-0.83%)	⬇️
reframe/core/config.py	82.6% <0%> (-1.74%)	⬇️

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[omlins]

Thanks @vkarak for the improvement. I changed the test-directory suffix back to the form _retry1 as agreed in the stand-up meeting. I believe the MR is ready for merging.