[feat] Support early access to fixtures

[vkarak]

The problem we have faced trying to implement the OSU benchmarks as a library in #2421 is that in the CSCS specialisation we wanted to set the valid_systems and valid_prog_environs based on the osu_binaries.build_type fixture parameter. This was only possible to achieve by creating explicitly different specialized tests for each build type, because we were not allowed to access fixtures before the setup stage. This PR solves this, allowing users to access fixtures as early as in a post-init hook. However, only fixture parameters are allowed to be accessed at this point, but it should be enough to serve us in almost all if not all scenarios. From the setup stage onward, there is no change in the behaviour compared to the past.

[codecov-commenter]

Codecov Report

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The diff coverage is 100.00%.

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@@            Coverage Diff            @@
##             master    #2484   +/-   ##
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  Coverage          ?   85.73%           
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  Files             ?       57           
  Lines             ?    10554           
  Branches          ?        0           
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  Hits              ?     9048           
  Misses            ?     1506           
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Impacted Files	Coverage Δ
reframe/core/pipeline.py	93.49% <ø> (ø)
reframe/core/fixtures.py	100.00% <100.00%> (ø)
reframe/core/meta.py	99.06% <100.00%> (ø)

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