Improve sru plugin

[JSCU-CNI]

Previously all sru plugin functions would throw an uncaught exception when the windows target does not contain a certain sru table. The plugin now gracefully exits with a warning when an sru table does not exist.

[Schamper]

We've also been meaning to change this plugin to use the information from the registry to dump available tables. But that's something for a different PR.

[Schamper]

Can you fix the unit tests?

[Schamper]

Looks like a different unit test doesn't like the year rollover 🙃 unfortunately we'll need to fix that first. The yum plugin uses the modification timestamp for the year rollover, but the CI probably changes the modification timestamp upon checkout.

(#492 to fix that)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (44c6613) 74.08% compared to head (27e16bf) 74.07%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #472      +/-   ##
==========================================
- Coverage   74.08%   74.07%   -0.01%     
==========================================
  Files         261      261              
  Lines       21110    21113       +3     
==========================================
+ Hits        15639    15640       +1     
- Misses       5471     5473       +2     

Flag	Coverage Δ
unittests	74.07% <100.00%> (-0.01%)	⬇️

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