tests: net: http_server: core: Fix scheduling race

After recent kernel changes there's some thread scheduling race when
running tests, therefore add k_yield() at the end of each test to make
sure the server thread has a chance to run and do the cleanup.

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

tests/net/lib/http_server/core/src/main.c

@@ -2750,6 +2750,8 @@ static void http_server_tests_after(void *fixture)
 	}
 
 	(void)http_server_stop();
+
+	k_yield();
 }
 
 ZTEST_SUITE(server_function_tests, NULL, NULL, http_server_tests_before,