Test coverage improvement for Wallet class

I have added few tests to `WalletTest` and related classes to improve test coverage there. In some cases I have also duplicated line coverage but added more asserts that check different things. Also updated `pom.xml` to exclude autogenerated files from being included in the code coverage report.
Surely this is tested already elsewhere? Wallet tx broadcast and propagation is what the mock broadcaster was written for, I think ...