Organise unit tests to use pytest fixtures (implement remaining files)

[codecov[bot]]

Codecov Report

Merging #178 (62d0ff8) into 163-tidy-up-organisation-of-unit-tests (eaa440b) will increase coverage by 8.52%. The diff coverage is 100.00%.

@@                            Coverage Diff                             @@
##           163-tidy-up-organisation-of-unit-tests     #178      +/-   ##
==========================================================================
+ Coverage                                   78.57%   87.10%   +8.52%     
==========================================================================
  Files                                          25       25              
  Lines                                        1601     1613      +12     
==========================================================================
+ Hits                                         1258     1405     +147     
+ Misses                                        343      208     -135     

Files	Coverage Δ
tests/test_fs.py	100.00% <100.00%> (+78.57%)	:arrow_up:
tests/test_pbs.py	100.00% <100.00%> (+80.00%)	:arrow_up:
tests/test_subprocess.py	100.00% <100.00%> (+93.75%)	:arrow_up:
tests/test_workdir.py	100.00% <100.00%> (+83.33%)	:arrow_up:

... and 3 files with indirect coverage changes

[SeanBryan51]

I think the design or logic in test_subprocess.py tests need to be rethink, but that's beyond this ticket.

@ccarouge can you make a new issue describing in more detail?