tests: net: tls_ext: Fix scheduling race

After recent kernel changes there's some thread scheduling race when
running tests, therefore add k_yield() at the end of the test to make
sure the network stack has a chance to run in between tests.
Make sure CONFIG_NET_TCP_TIME_WAIT_DELAY is set to 0 so that TCP
connections are released immediately.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>