tests: add mkdirat.test

* tests/mkdirat.c: New file.
* tests/mkdirat.test: New test.
* tests/.gitignore: Add mkdirat.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add mkdirat.test.
diff --git a/tests/.gitignore b/tests/.gitignore
index c488409..ea67040 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -73,6 +73,7 @@
 membarrier
 memfd_create
 mincore
+mkdirat
 mknod
 mlock2
 mmap