Convert all Storable's tests to use Test::More.

Convert all Storable's tests to use Test::More.

Originally Storable didn't use any test modules, and had an ok subroutine in
t/st-dump.pl. Subsequently some tests were made conditional on Test::More
loading, and more recently the distribution started bundling Test::More, at
which point newer tests were written to use it. However, the older tests have
never been refactored to use it. Hence refactor tests to use Test::More, and
delete the now-unused test functions from t/st-dump.pl

Tested on blead and 5.004.