test: make test-condvar call uv_cond_wait

Problem:
The condvar tests could pass without uv_cond_wait
(or uv_cond_timedwait) ever being invoked.

Solution:
Introduce semaphores to enforce ordering.
Now there will always be a thread waiting on the condition
when a signal() occurs.

Gotchas:
1. On Windows, waiting for a timeout may return earlier
than requested, depending on the granularity of timer ticks.
2. Timeout bounds are tuned based on our CI machines.

Bonuses:
1. I added additional test cases to complete the test matrix.

2. It seemed to me that several of the condvar tests were redundant,
because they used timing to probabilistically explore cases where there
would be "missed connections" (signal without a waiter).
Because it was not clear to me what the purpose of such tests were,
I have removed them.

Fixes: https://github.com/libuv/libuv/issues/1714
PR-URL: https://github.com/libuv/libuv/pull/1718
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

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