[feat] Add sanity_function and deferrable builtins

[jjotero]

This PR introduces the sanity_function and deferrable decorators, which are exposed directly in the class' namespace.

• A test's sanity function may now be set as follows:

class MyTest(rfm.RunOnlyRegressionTest):
    ...
    executable = 'echo bananas'

    @sanity_function
    def assert_bananas(self):
        return sn.assert_found(r'bananas', self.stdout)

The gpu microbenchmarks from the hpctestlib (and also the job report check) have been ported to this syntax. Using this new syntax in combination with setting sanity_patterns manually is disallowed.

• The deferrable built-in decorator exposes the same functionality as the reframe.utility.sanity.sanity_function decorator.
• The reframe.utility.sanity.sanity_function has been marked to be deprecated in 3.8.
• This same decorator is now exposed as reframe.utility.sanity.deferrable.

Closes #1981
Closes #2008

[codecov-commenter]

Codecov Report

Merging #2007 (a02037e) into master (c5f59fa) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2007      +/-   ##
==========================================
+ Coverage   87.48%   87.51%   +0.02%     
==========================================
  Files          50       50              
  Lines        8853     8874      +21     
==========================================
+ Hits         7745     7766      +21     
  Misses       1108     1108              

Impacted Files	Coverage Δ
reframe/core/meta.py	98.89% <100.00%> (+0.09%)	⬆️
reframe/core/pipeline.py	91.68% <100.00%> (+0.05%)	⬆️
reframe/utility/sanity.py	97.79% <100.00%> (+0.01%)	⬆️

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[ekouts]

lgtm in general

[vkarak]

I have just a couple of minor comments. Lgtm otherwise.

[vkarak]

lgtm