46ce223d1 Add tests for CMerkleBlock usage with txids specified (James O'Beirne)
5ab586f90 Consolidate CMerkleBlock constructor into a single method (James O'Beirne)
Pull request description:
What started as a simple task to add test coverage ended up giving way to a light refactoring. This consolidates the mostly-identical `CMerkleBlock` constructors into one (using C++11 constructor delegation) and adds coverage for the by-txids construction case.
### Before

### After

Tree-SHA512: eed84ed3e8bfc43473077b575c8252759a857e37275e4b36ca7cc2c17a65895e5f494bfd9d4aeab09fc6e98fc6a9c641ac7ecc0ddbeefe01a9e4308e7909e529
Backport of Core PR11293
https://github.com/bitcoin/bitcoin/pull/11293/
Adjusted to take into account D3371