pytest:segfault: prevent @no_gdb_backtrace smearing on exception

pytest:segfault: prevent @no_gdb_backtrace smearing on exception

It is OK for one of these tests to raise an exception -- that is often
the only reasonable thing to do when you'd otherwise crash -- but the
@no_gdb_backtrace decorator would not clean up in that case, leading to
no gdb backtraces for all subsequent tests.

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