unified/test_context: adapt test to run on unified kernel

Fixing issues with the test itself really:

- reply_timeout semaphore was not initialized, causing its limit value
  to be 0 on unified kernel

- There is no API to set a fiber's priority after it is started in a
  nanokernel. However, tcs.prio can be written to and this works without
  issues. On unified kernel, this does not work however because the
  thread has to move between linked lists representing each priority in
  the ready queue.

Change-Id: I3c5585da05cbc4ac3d2f0f9ae0297d24d41b1309
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>