[enhancement] Print the fixture variable names in test listings

[vkarak]

When listing fixtures, we also print the variable name to which they are bound, so that users are able to select them in the -S option without having to inspect the test file. I have also enhanced the docs. This is how the new listing looks like:

- osu_latency_test /d2c978ad
    ^build_osu_benchmarks ~generic:default+builtin 'osu_binaries /5cf701b0
      ^fetch_osu_benchmarks ~generic 'osu_benchmarks /9fc7952e

Closes #2763.

[codecov-commenter]

Codecov Report

Base: 86.61% // Head: 86.57% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (23554ad) compared to base (d74dd1f).
Patch coverage: 30.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2783      +/-   ##
==========================================
- Coverage   86.61%   86.57%   -0.05%     
==========================================
  Files          60       60              
  Lines       11218    11226       +8     
==========================================
+ Hits         9717     9719       +2     
- Misses       1501     1507       +6     

Impacted Files	Coverage Δ
reframe/frontend/cli.py	73.22% <30.00%> (-0.62%)	⬇️

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