tests: add unlink.test

* tests/unlink.c: New file.
* tests/unlink.test: New test.
* tests/.gitignore: Add unlink.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add unlink.test.
diff --git a/tests/.gitignore b/tests/.gitignore
index 88dd092..72a2e1d 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -226,6 +226,7 @@
 umovestr2
 uname
 unix-pair-send-recv
+unlink
 unlinkat
 userfaultfd
 utime