Add test for CWalletTx::GetImmatureCredit() returning stale values.

Summary:
Add test for cached immature credit flag not being cleared in
CWalletTx::MarkDirty() bug, which was fixed in
https://github.com/bitcoin/bitcoin/pull/8717, commit a560378.

Bacport of core's PR9359

Test Plan:

make check

Reviewers: schancel, #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D941