test-utils: Resolve control reaching the end of a non-void function

Resolves:

    tests/test-utils.c: In function ‘mctp_binding_test_tx’:
    tests/test-utils.c:20:1: error: no return statement in function returning non-void [-Werror=return-type]
       20 | }
          | ^
    cc1: all warnings being treated as errors

Functions terminating in an assert() won't always have the assert()
present to terminate them.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Ieb4338c994675e50b020962d22cff338e598e43a
diff --git a/tests/test-utils.c b/tests/test-utils.c
index df04343..fa7f460 100644
--- a/tests/test-utils.c
+++ b/tests/test-utils.c
@@ -17,6 +17,7 @@
 {
 	/* we are not expecting TX packets */
 	assert(0);
+	return 0;
 }
 
 struct mctp_binding_test *mctp_binding_test_init(void)