tests: check decoding of setfsgid syscall

* tests/setfsugid.c: New file.
* tests/setfsgid.c: New file.
* tests/setfsgid.test: New test.
* tests/.gitignore: Add setfsgid.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add setfsgid.test.
(EXTRA_DIST): Add setfsugid.c.
diff --git a/tests/.gitignore b/tests/.gitignore
index 1429c19..cd19268 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -142,6 +142,7 @@
 sendfile
 sendfile64
 set_ptracer_any
+setfsgid
 setfsuid
 setfsuid32
 setgid