tests: extend test coverage of getresgid syscall

* tests/getresgid.c: New file.
* tests/getresgid.test: New test.
* tests/.gitignore: Add getresgid.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add getresgid.test.
diff --git a/tests/.gitignore b/tests/.gitignore
index 9c46400..85b6357 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -62,6 +62,7 @@
 getgroups
 getgroups32
 getrandom
+getresgid
 getresuid
 getresuid32
 getrusage