selftest: make filter-subunit much more efficient for large knownfail lists

selftest: make filter-subunit much more efficient for large knownfail lists

By compiling the knownfail lists ahead of time we change a 20min test
into a 90sec test.

This could be improved further by combining this into a single regular expression,
but this is enough for now.  The 'reason' is thankfully not used.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15258

Pair-programmed-with: Joseph Sutton <josephsutton@catalyst.net.nz>

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
(cherry picked from commit 22128c718cadd34af892df102bd52df6a6b03303)