Fix long-standing bug in t/op/inc.t, whereby ok() ignored a failed match.

Fix long-standing bug in t/op/inc.t, whereby ok() ignored a failed match.

Unlike test.pl and Test::More, the home-rolled ok() in t/op/inc.t didn't have
a prototype. Hence its arguments are in *list* context, meaning that any match
will return an *empty list* if it fails. Provided ok() was called with a
second, true, parameter, the failed match would not be noticed, because ok()
would register a test pass, because it would now be testing the (intended)
second parameter.

Add a prototype, and fix the logic for the tests affected.
Fortunately this wasn't concealing any bugs.