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.
7 files changed