[feat] Add support for marking tests as expected failures

[vkarak]

The main elements introduced by this PR are:

1. The @xfail decorator to mark a test as an expected sanity failure.
2. The xfail() builtin to mark reference tuples as expected performance failures.

Two new test states are introduced which are also logged: XFAIL for expected failures and XPASS for unexpected passes.

Design-wise the concept is the same as the one explained in #2378.

Implementation-wise there are two major changes:

1. The execution policies code is refactored by eliminating code duplication between the serial and asynchronous policies.
2. A new utility function reference_bounds() is introduced which is used to validate and calculate the absolute lower/upper bounds. This used to be internal to assert_reference() and now it's made public (maybe the docs there need a slight reorganization).
3. The treatment of performance failures is rewritten as now we have to handle much more complex cases, were the performance variables of a test can simultaneously be pass, fail, xpass or xfail.

For more details on how feature works macroscopically check the tutorial and the reference docs.

Closes #2378.

[victorusu]

lgtm.

[codecov]

Codecov Report

Attention: Patch coverage is 75.11111% with 56 lines in your changes missing coverage. Please review.

Project coverage is 67.14%. Comparing base (2a2dd82) to head (489e3b0).

Files with missing lines	Patch %	Lines
reframe/frontend/executors/__init__.py	60.46%	17 Missing ⚠️
reframe/frontend/executors/policies.py	80.55%	14 Missing ⚠️
reframe/core/decorators.py	0.00%	11 Missing ⚠️
reframe/core/pipeline.py	93.50%	5 Missing ⚠️
reframe/core/builtins.py	25.00%	3 Missing ⚠️
reframe/utility/sanity.py	57.14%	3 Missing ⚠️
reframe/core/exceptions.py	0.00%	2 Missing ⚠️
reframe/frontend/printer.py	66.66%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (2a2dd82) and HEAD (489e3b0). Click for more details.

HEAD has 17 uploads less than BASE

Flag	BASE (2a2dd82)	HEAD (489e3b0)
	22	5

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #3481       +/-   ##
============================================
- Coverage    91.13%   67.14%   -24.00%     
============================================
  Files           62       60        -2     
  Lines        13132    13167       +35     
============================================
- Hits         11968     8841     -3127     
- Misses        1164     4326     +3162     

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