Make filetest ops fiddle the stack less

Make filetest ops fiddle the stack less

See commits 8db8f6b697e and d6642e439 for the background.

Instead of sometimes popping items off the stack for filetest operat-
ors and sometimes not, just leave the item on the stack and use SETs
to return the value (which the overload code was doing already any-
way).  This simplifies the code and allows d6642e439 to be reverted.
It also makes things theoretically faster, simply because there is
less stuff happening.