tests: add a test for sigreturn/rt_sigreturn decoding

Convert test/sigreturn.c into a regular test.

* test/Makefile (PROGS): Remove sigreturn.
* test/.gitignore: Likewise.
* test/sigreturn.c: Rewrite to ...
* tests/sigreturn.c: ... new file.
* tests/sigreturn.test: New test.
* tests/Makefile.am (check_PROGRAMS): Add sigreturn.
(TESTS): Add sigreturn.test.
* tests/.gitignore: Add sigreturn.
diff --git a/tests/.gitignore b/tests/.gitignore
index a55b556..8efb0ef 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -15,6 +15,7 @@
 select
 set_ptracer_any
 sigaction
+sigreturn
 stack-fcall
 stat
 stat32