[test] call CheckPackage for package sanitization checks

Summary:
Makes the test more minimal. We're just trying to test that our package
sanitization logic is correct. Now that this code lives in its own
function (rather than inside of AcceptMultipleTransactions), there's no
need to call ProcessNewPackage to test this.

This is a partial backport of core#23381
https://github.com/bitcoin/bitcoin/pull/23381/commits/14cd7bf793547fa5143acece564482271f5c30bc
Depends on D12361

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12362