tests: Fix eina_thread_queue test case 5 & 6

tests: Fix eina_thread_queue test case 5 & 6

ecore_test_ecore_thread_eina_thread_queue_t6 failed often for me.
eina_thread_queue_wait() was returning NULL.

I believe this is because the test case ended abruptly without
waiting for the threads to finish. Indeed, both threads tried
hard to reach 10000 messages but it didn't make sense for them
both to reach this value, only one would end there.

This patch adds an exit message sent by thread 1 to the two other
threads, and all threads are waited upon using a single semaphore.

Note: This also renames some functions to match their test case
number.

@raster