selftest/subunithelper: only let ']\n' lines to terminate

selftest/subunithelper: only let ']\n' lines to terminate

It should not be enough that a line ends with ']\n' is accident,
subunit DETAILS are terminated with '\n]\n'!

This gives a much higher chance to see the actual problem
without having them filtered by various 'filter-subunit' invocations.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>