Fix for bad-switch test failing on Darwin; system-specific output.

Because the 'getopt' library differs between systems, it's likely
that there will be different output. This is the case between the
GNU-based Linux and the BSD-based Darwin, for the 'getopt' library,
it seems. It causes the tests to produce different results.

To allow us to test, and check for regressions, on the different
platforms, the invocation code has been updated to allow a
system-specific suffix to be used on the test files. If this suffix
is found, the test will also be flagged as being system-specific
which should ensure the change in behaviour is noted.

tests/invocation/bad-switch.err.Darwin

@@ -0,0 +1,2 @@
+fish: invalid option -- Z
+RC: 1