test(utils): wait for the read only lock to register before probing - #4943

PrioritiseReadOnlyLock slept 15ms after its latch, hoping the deferred
thread had reached lock() and registered as pending, then probed once
with a WRITE try_lock. Under CPU load the probe lands first, sees a
gate that is legitimately still open, acquires, and fails the test.

The probe now retries until the gate is observably closed. A WRITE
refused here can only be the pending READ_ONLY, since the WRITE the
test already holds is compatible with another WRITE.

The failure also hung the binary: the assertion returned from the test
body while the deferred thread was still parked in lock(), and only
the release the test never reached could free it, so ~future blocked
until the job timeout. The release now happens before the result is
reported.

The worker also asks for std::launch::async explicitly. Without it a
deferred launch runs nothing until get(), and the test blocks on a
latch whose other arrival never comes.

Closes #4941

The retry loop waited up to 60s for the deferred thread to register as
pending. The whole binary shares a 120s ctest budget across 37 tests, so
a broken lock spent half of it spinning before reporting. The wait is one
mutex acquisition, and 5s matches the watchdog bounds the rest of the file
already uses.