fusefs: fix a race in the pre-init tests

These tests allow the user to customize the INIT response. But it's
necessary to block the daemon's service loop from running until those
expectations have been set. This race has never caused failures before
simply due to luck. But now it's failing on slower platforms.

PR: 296236
Reported by: siva
MFC after: 2 weeks
Sponsored by: ConnectWise
Reviewed by: siva
Differential Revision: https://reviews.freebsd.org/D57781