While this regression test "passes", this PR resolves two errors to actually compare concentrations results:

1. Fix typo for filename "gwt_mst03.unc" ->"gwt_mst03.ucn"
2. Compare the two concentration files with compare_heads with text="concentration".

Further explanations to the fixes:

1. The flopy.utils.compare module "passes" (returns assert True) if there are there are not two files to compare, which is an unusually silent approach. This was due to the filename typo.
2. These files are not MT3D-style concentration files, and would raise exceptions if read with UcnFile. They must be read with HeadFile(..., text="concentration")