test-concurrency: Don't lower timeout

test-concurrency: Don't lower timeout

I think there's never been a real race condition here.  Instead
the problem is:

- We have a timeout on the lock, after which we error out (30s)
- This test actually *lowers* the timeout

Looking through the failures for test-concurrency what I see
is
`error: Locking repo exclusive failed: Resource temporarily unavailable`
which is us hitting the timeout.

Hardcoded timeouts are just going to be subject to race conditions.
I understand not wanting to block forever in some cases, but any
arbitrary timeout is just going to get hit in real world conditions
too.

Anyways for now, stop shooting ourselves in the foot and at least
keep the timeout at the default.

Closes: https://github.com/ostreedev/ostree/issues/2038