tests: lib: fdtable: Add tests for checking refcounting

Add tests that will make sure that refcounting works as expected.

Also fixed two tests that were using the object values before
the fdtable contained proper values. After reserving a fd by
z_reserve_fd(), we can only call z_finalize_fd() or z_free_fd(),
and only after those calls can fetch obj and vtable values.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>