tests: add getsid.test

* tests/getsid.c: New file.
* tests/getsid.test: New test.
* tests/.gitignore: Add getsid.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add getsid.test.
diff --git a/tests/.gitignore b/tests/.gitignore
index 2f5f95e..d89e9e6 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -73,6 +73,7 @@
 getresuid
 getresuid32
 getrusage
+getsid
 getuid
 getuid32
 getxxid