Ensure that we test the event condition when a disabled kevent is enabled.

r274560 modified kqueue_register() to only test the event condition if the
corresponding knote is not disabled. However, this check takes place before
the EV_ENABLE flag is used to clear the KN_DISABLED flag on the knote, so
enabling a previously-disabled kevent would not result in a notification for
a triggered event. This change fixes the problem by testing for EV_ENABLED
before possibly checking the event condition.

This change also updates a kqueue regression test to exercise this case.

PR: 206368
Reviewed by: kib
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D5307