SubdocPerfTest: Minimise iterations when run under sanitizers
SubdocPerfTest: Minimise iterations when run under sanitizers

When running under TSan, ASan, UBSan or Valgrind only run
SubdocPerfTest for a minimal number of iterations.

Reduces the wallclock runtime for this suite under ASan-UBSan from 95s
to ~2s on mancoucn.

Change-Id: I1e82b19ca38fd81000c0a15bdea69d1d902be389
Reviewed-on: http://review.couchbase.org/115765
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine