selftest: Fail testsuite on unknown environment

selftest: Fail testsuite on unknown environment

The status quo was that tests in typo'd/unknown environments would be skipped.
This was a side effect of the mechanism by which samba3 tests were run, which
has now been changed.

For an example of this being a problem in the past, see commit 523bd03f (which
fixes an issue I noticed when reading subunit logs, and which did not fail the
test).

Signed-off-by: Jamie McClymont <jamiemcclymont@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>