libevent: Increase ptest timing tolerance 50 ms -> 100 ms

Adjusting the tolerance to a more reasonable time
given the load on the AB and given the high amount(100) of
events some of the tests like `common_timeout` generates.

[YOCTO #14163]

Signed-off-by: Yi Fan Yu <yifan.yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>