unified/test_mutex: adapt to run on unified kernel

- Use a more limited range of priorities, since the current
  implementation of the unified kernel only works with 32 priorities
  total. Instead of starting at 10 and going up by 5 up to  50, start
  at 5 and go up by 1 up to 12.

- The definition of kmutex_t has changed from a uint32_t to a struct
  k_mutex *, causing this to not work anymore:

    const kmutex_t private_mutex;

  since this makes the object constant instead of the reference to it.
  Private object must be referenced like this instead:

    kmutex_t const private_mutex;

  since const is left-associative.

Change-Id: I9d70bfa3944ea46033a6b49251a4993e9bd2b588
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>