tests: extend test coverage of setresgid syscall

* tests/setresgid.c: New file.
* tests/setresgid.test: New test.
* tests/.gitignore: Add setresgid.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add setresgid.test.
diff --git a/tests/.gitignore b/tests/.gitignore
index b0698fa..c2ac5c8 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -147,6 +147,7 @@
 sethostname
 setregid
 setregid32
+setresgid
 setresuid
 setresuid32
 setreuid