[feat] Allow pinning a hook as the last of its stage

[vkarak]

This PR introduces a new argument always_last to the @run_before and @run_after decorators that will pin the current hook as the last of its stage. Only one such hook can exist for each stage in a test hierarchy. This feature should make writing test libraries easier, as they can pin their hooks at the end of a stage overriding therefore therefore the default hook execution order.

Closes #2749.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (0667c83) 87.30% compared to head (574387b) 87.31%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2992      +/-   ##
===========================================
+ Coverage    87.30%   87.31%   +0.01%     
===========================================
  Files           60       60              
  Lines        11696    11708      +12     
===========================================
+ Hits         10211    10223      +12     
  Misses        1485     1485              

Files Changed	Coverage Δ
reframe/core/builtins.py	100.00% <100.00%> (ø)
reframe/core/hooks.py	90.21% <100.00%> (ø)
reframe/core/pipeline.py	94.02% <100.00%> (+0.09%)	⬆️

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[victorusu]

lgtm
I wonder if we can have dedicated documentation about it.
I don't think it should come in this PR, but a session explaining this feature would be nice.
What do you think?

[vkarak]

I wonder if we can have dedicated documentation about it.
I don't think it should come in this PR, but a session explaining this feature would be nice.

Yes, it makes sense. I'll have a look in the tutorials if this could be showed in the examples we already have. Otherwise we will have to address this separately.