add a simple extra test to the steady state solver

[Krastanov]

I needed this test as I was debugging https://github.com/qojulia/QuantumOpticsBase.jl/issues/106 -- it is nothing sophisticated, just making the already present tests a bit more explicit

[codecov[bot]]

Codecov Report

Merging #365 (5bbfe4f) into master (03ccf40) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #365   +/-   ##
=======================================
  Coverage   98.05%   98.05%           
=======================================
  Files          17       17           
  Lines        1442     1442           
=======================================
  Hits         1414     1414           
  Misses         28       28           

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