Bump the time limit for valgrind fuzzing

Some captures are still hitting the limit as a false positive.

Bug: 10122
Change-Id: I12dd26ba0f96f44799f5ae642b1f9b0487f86a8a
Reviewed-on: https://code.wireshark.org/review/3982
Reviewed-by: Michael Mann <mmann78@netscape.net>