[rpc] Return fee and size from testmempoolaccept

Summary:
Return fee and size if tx would pass ATMP.

This is a backport of core#19940

D1635 introduced CTransaction.billable_size, so use it instead of adding a new identical get_size method.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10279