libbinder_ndk_unit_test: wait slightly longer

If the timing is unlucky:
- servicemanager might have 'guaranteeClient' set for 5s
- then the timerfd will expire 5s after that

Due to scheduler delays, the service may not be shutdown after 5s.

Bug: 243452512
Change-Id: I7f0377b0d835900bfef8e0ec55389843357f779d
Test: libbinder_ndk_unit_test