[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 97.77778% with 5 lines in your changes missing coverage. Please review.

Project coverage is 91.16%. Comparing base (2a2dd82) to head (17d6122).
Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
reframe/frontend/executors/__init__.py	93.02%	3 Missing ⚠️
reframe/core/decorators.py	90.90%	1 Missing ⚠️
reframe/frontend/executors/policies.py	98.61%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## develop #3481 +/- ## =========================================== + Coverage 91.13% 91.16% +0.02%  =========================================== Files 62 62 Lines 13132 13253 +121 =========================================== + Hits 11968 12082 +114  - Misses 1164 1171 +7 

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