tests: Relax the requirement of bfd tests on timing.
Before this commit, the bfd related tests check the change of
states with very strict timing requirement. This makes the tests
fragile, especially when ofproto-dpif-monitor module is moved to
its own thread.
Thusly, this commit relaxes the requirement on timing, making the
tests more reliable.
Signed-off-by: Alex Wang <alexw@nicira.com> Signed-off-by: Ethan Jackson <ethan@nicira.com> Acked-by: Ethan Jackson <ethan@nicira.com>