Added a test for wake_with(). It tries to ensure that the problematic
case solved by wake_with() actually happens quickly, by:
 1. Spin a long time between the setting of the flag and t->wake()
 2. Do a spurious wake() to ensure that the waiting thread is woken
    up right after setting the flag, before the intended wake.
 3. Use mprotect() to ensure that working with an already join()ed
    thread crashes immediately, instead of just maybe crashing.

This test fails when wake_with() doesn't use ref()/unref(), and succeeds
with the full wake_with().

tst-wake contains a second test, which does the same thing but without
the additional measures we used to show the bug (spinning, spurious
wake and mprotect). Without these additional measures the test iteration
is much faster, which allows us to stress wake/join much more.