[utest] Cleanup: Make condvar test pass if run multiple times

This is useful for running the test in a loop as a stress test (though
additional changes are required to do that).

Change-Id: I222f2db67203af8e892c1e94d6e1011cbf9cfd25

[utest] Condvar test: Speed up the test by reducing the timeout

Wait for just 1 ms instead of 2 seconds.

Change-Id: Ibc62916b08b9ed1c5b509fe6a88e788e78584f14

[utest] Condvar test: Make test stricter: check for specific result code

Change-Id: Ie7eb2e48d00cae517ebde1cee0df23fa764a04cb

[utest] Condvar test: Factor out some duplication

Convert the three cnd_signal() cases into a loop.

Change-Id: I78a64be092b333bbe0ebaad5f8329357efc46279

[utest] Condvar test: Move timeout test into a separate test case

This means that if any of these tests fail or hang, it will be clearer
which part is failing/hanging.

Change-Id: I667aa946dd78f557a9f52ea064f738a138a650ca

[cnd_test] Fix race in cnd_test

Previously there was a race in which the main thread broadcasted
and then signaled before any of the threads woke.

Change-Id: I219efbf58f1fadf7cbd1d1ac3f0b5d4df46171a8

[musl][cnd_t] Fix cnd_timedwait

The original implementation of cnd_t relied on mtx_t having the same
layout as pthread_mutex_t. However, we have changed mtx_t to be
considerably smaller and simpler, based on mxr_mutex_t. Accordingly,
this patch essentially recreates the cnd_timedwait implementation, with
the internal locking changed. It is essentially the same as the
pthread_cond_timedwait implementation, with some dead code removed due
to the simpler mutex implementation.

Change-Id: If56f3be4547d16a567dfa9cda0954430c126a52c