tests: shell: add missing assert

Add missing assert after polling for the dummy shell backend to be ready.

Fixes: 491f4dd701

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>

tests/subsys/shell/shell/src/main.c

@@ -495,6 +495,7 @@ static void *shell_setup(void)
 
 	/* Wait for the initialization of the shell dummy backend. */
 	WAIT_FOR(shell_ready(sh), 20000, k_msleep(1));
+	zassert_true(shell_ready(sh), "timed out waiting for dummy shell backend");
 
 	return NULL;
 }