Merge #11714: [tests] Test that mempool rejects coinbase transactions

Summary:
65e91f5ed [tests] Test that mempool rejects coinbase transactions (James O'Beirne)

Pull request description:

![selection_063](https://user-images.githubusercontent.com/73197/32978622-b0fa9d70-cbfa-11e7-9a72-1997409e5ba8.png)

Neither the unit nor functional tests appear to cover rejecting a transaction from acceptance to the mempool on the basis of it being a coinbase. Seems like a decent thing to have a test for.

Tree-SHA512: 53af53c975cad5d7a21c443d71a1c0ced5c70a7799b75bb44d9b7dd6ab2afbcdcaab14571540efeb848f3a1daee5e1dd856530d8f2b50582595219a1c17555ff

Backport of Core PR11714
https://github.com/bitcoin/bitcoin/pull/11714/

Test Plan:

make check
ninja check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3697