Check that CheckRegularTransaction rejects coinbases and vice-versa

Summary: This is somewhat of a hole in the test plan.

Test Plan:

make check

Remove the coinbase check from CheckRegularTransaction and make sure that everything fails.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1867