…le (#19478)

The flake here had two causes:
1. related to usage of time.Now() in MustWaitForProvisionersAvailable
and
2. the fact that UpdateProvisionerLastSeenAt can not use a time that is
further in the past than the current LastSeenAt time

Previously the test here was calling
`coderdtest.MustWaitForProvisionersAvailable` which was using `time.Now`
rather than the next tick time like the real `hasProvisionersAvailable`
function does. Additionally, when using `UpdateProvisionerLastSeenAt`
the underlying db query enforces that the time we're trying to set
`LastSeenAt` to cannot be older than the current value.

I was able to reliably reproduce the flake by executing both the
`UpdateProvisionerLastSeenAt` call and `tickCh <- next` in their own
goroutines, the former with a small sleep to reliably ensure we'd
trigger the autobuild before we set the `LastSeenAt` time. That's when I
also noticed that `coderdtest.MustWaitForProvisionersAvailable` was
using `time.Now` instead of the tick time. When I updated that function
to take in a tick time + added a 2nd call to
`UpdateProvisionerLastSeenAt` to set an original non-stale time, we
could then never get the test to pass because the later call to set the
stale time would not actually modify `LastSeenAt`. On top of that,
calling the provisioner daemons closer in the middle of the function
doesn't really do anything of value in this test.

**The fix for the flake is to keep the go routines, ensuring there would
be a flake if there was not a relevant fix, but to include the fix which
is to ensure that we explicitly wait for the provisioner to be stale
before passing the time to `tickCh`.**

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Signed-off-by: Callum Styan <callumstyan@gmail.com>