tests: lib: fdtable: fix tests after modifying reference counting

Tests need to be adjusted to do the correct checks now that reference
count is incremented in z_reserve_fd() instead of z_finalize_fd().

test_z_fd_multiple_access in particular has been simplified to
reserving an fd in one thread and freeing it in the other.

Signed-off-by: Vincent Wan <vwan@ti.com>