tests: make test executables invocable outside current work directory

tests: make test executables invocable outside current work directory

Most of test executables were ready for the upcoming change,
this change prepares all the rest.

* tests/getdents.c (main): Use fixed name for the sample directory.
* tests/getdents64.c (main): Likewise.
* tests/readdir.c (main): Likewise.
* tests/mknod.c (sample): Change to a pointer.
(main): Use av[0] as a sample.
* tests/symlink.c (main): Use av[0] as a linkpath.
* tests/umode_t.c (sample): Change to a pointer.
(test_syscall): Fix expected output.
(main): Use av[0] as a sample.