Add function to unregister IUTFMonitor

[MichaelHuth]

Function UnRegisterIUTFMonitor()

    DFREF dfr = GetPackageFolder()
    variable/G dfr:BCKG_Registered = 0

    CtrlNamedBackground $BACKGROUNDMONTASK stop
End

Sometimes within a test case one could decide to not to do reentry, but reentry is default setup in e.g. TEST_CASE_BEGIN.

[t-b]

If we add runtime skip support as suggested in #467, would we automatically unregister? I would think yes.

[MichaelHuth]

Yes, skip should unregister automatically. Still I think it would make sense to have that function callable from a TC.

[t-b]

Fixed via !469.