tests: net: context: Let the net_context cb to run first

An issue was seen after changes introduced in #17933.
The net_context callback was run after we checked that it
was run ok. This test failed of course in that case. Simple
solution is to k_yield() which will make sure that the
callback gets called before we continue. This works for this
test as our threads have suitably selected priorities.
There is also no need to use K_NO_WAIT here so replacing the
timeout with K_FOREVER.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>