Attribute-Handlers: Swap out base for parent in test files.

Committer's note: Add dist/Attribute-Handlers/demo/MyClass.pm to list of
files whose $VERSION numbers do not have to be incremented when modified.
Since this file is found in a 'demo/' directory, it does not have to be
treated as if it were a real Perl library which happens to be shipping
with core.

For: RT #119365