tdb2: simplify failtest helper.

tdb2: simplify failtest helper.

failtest now culls duplicates for itself (and more efficiently), so
don't replicate the logic here.  It changes things a bit, because
failtest uses backtraces rather than a simple call point to find
duplicates.

Also, fix one case (in run-11-simple-fetch.c) where we simply exited
rather than using failtest_exit().  We got away with it before, because
we never hit that particular failure pattern.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
(Imported from CCAN commit 3d99c9334fe3067c88772547b9c06acec21616ea)