selftest: Abort if we fail to startup testenv with '--one' option

selftest: Abort if we fail to startup testenv with '--one' option

The --one selftest.pl option means abort when the first test fails.
However, when 'make test' fails to startup a testenv, it'll try to
continue and run other tests by default. When '--one' is used,
selftest.pl can just die() at that point.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>