Improve the BEGIN-time setup code for File::Find's tests.

In find.t merge the two BEGIN blocks and eliminate the redundant
C<use File::Spec;>
In both, don't attempt to change directory to t/ (which will be unhelpful
once File::Find is moved to ext/)
Only make paths in @INC absolute if $ENV{PERL_CORE} is set (which will ease
making File::Find dual-life, if we want to this.)
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.
Move File::Find from lib/ to ext/