tests: e1inp_ipa_*_test: fix leak of msgb in ->sign_link path

Fix a leak in the tests: The ->sign_link callback is reponsible
for releasing the msgb.
diff --git a/tests/e1inp_ipa_bts_test.c b/tests/e1inp_ipa_bts_test.c
index f885c5c..31aac95 100644
--- a/tests/e1inp_ipa_bts_test.c
+++ b/tests/e1inp_ipa_bts_test.c
@@ -151,6 +151,7 @@
 		LOGP(DBTSTEST, LOGL_ERROR, "Unknown signalling message.\n");
 		break;
 	}
+	msgb_free(msg);
 	return ret;
 }