test: simplify for clarity

test: simplify for clarity

This should help avoid further audit confusion,
such as was just fixed by removing a FIXME.
* src/test.c (enum binop): New type.
(get_mtime): Return a struct timespec instead of returning
a bool and storing a struct timespec.  All callers changed.
(binop): Return an int recording either success (an enum binop)
or failure (-1).  All callers changed.
(binary_operator): Accept an enum binop, so that we need
not recompute the op type.  All callers changed.  Simplify.