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

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.)