Allow TAP tests to run under Msys

The Msys DTK perl, which is required to run TAP tests under Msys as a
native perl won't recognize the correct virtual paths, has its osname
recorded in the Config module as 'msys' instead of 'MSWin32'. To avoid
having to repeat the test a variable is created that is true iff the
osname is either of these values, and is then used everywhere that
matters.