Remove dSP from all filetest ops.

Remove dSP from all filetest ops.

Following commit d2c4d2d1e22d3125, all filetest ops avoid manipulating the
stack pointer until the point of pushing a return value. As all the filetest
returns now go through either FT_RETURN_FALSE() or FT_RETURN_TRUE(), it's
viable to put the dSP inside those two macros and use *PL_stack_sp in place
of TOPs. This eliminates all uses of sp in the bodies of the functions
(explicit, or implicit via macros), and that makes it clear that the main
bodies of the functions are not manipulating the stack.