[doc] Update figure and text about test execution

[vkarak]

This is how the figure and the text look like now:

Fixes #1983.

[jjotero]

lgtm

[teojgo]

@jenkins-cscs retry eiger

[codecov-commenter]

Codecov Report

Merging #2002 (8a7b294) into master (41d5492) will decrease coverage by 0.15%.
The diff coverage is 89.24%.

@@            Coverage Diff             @@
##           master    #2002      +/-   ##
==========================================
- Coverage   87.61%   87.46%   -0.16%     
==========================================
  Files          50       50              
  Lines        8788     8850      +62     
==========================================
+ Hits         7700     7741      +41     
- Misses       1088     1109      +21     

Impacted Files	Coverage Δ
reframe/core/decorators.py	64.70% <25.00%> (-18.49%)	⬇️
reframe/core/hooks.py	92.78% <92.00%> (-1.82%)	⬇️
reframe/core/meta.py	98.80% <96.66%> (-1.20%)	⬇️

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