Change File::Find's tests to look for taint.t instead of commonsense.t

Change File::Find's tests to look for taint.t instead of commonsense.t

This passes whether the tests are in lib/ (and hence run with the CWD as t/)
or in ext/File-Find (and hence run with that as the CWD) because they find
t/op/taint.t or ext/File-Find/t/taint.t respectively.

Change taint.t to count the number of times it finds a file named taint.t,
and fail if the count isn't 1. find.t was already testing a count.