tests: check silent decoding of sendmmsg and recvmmsg syscalls

* tests/mmsg-silent.c: New file.
* tests/mmsg-silent.test: New test.
* tests/.gitignore: Add mmsg-silent.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add mmsg-silent.test.
diff --git a/tests/.gitignore b/tests/.gitignore
index fa20bbd..06651bd 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -138,6 +138,7 @@
 mmap
 mmap64
 mmsg
+mmsg-silent
 mmsg_name
 mmsg_name-v
 mount