Merge #10295: [qt] Move some WalletModel functions into CWallet

Summary:
108f04f Add missing LOCK2 in CWallet::GetAvailableBalance (Russell Yanofsky)
429aa9e [test] Move some tests from qt -> wallet (Russell Yanofsky)
d944bd7 [qt] Move some WalletModel functions into CWallet (Russell Yanofsky)
ef8ca17 [test] Add tests for some walletmodel functions (Russell Yanofsky)

Tree-SHA512: f6384d9f2ff3f7fb173d414588c3e7dc8c311b8ed2ce2b0979fb824a0ed83a7302890ccd3d83197f07f6fdcb6b1ca151584d90ea1961d88dfe8956c87087cde8

Backport of Core PR10295
Completes T516

Test Plan:
make check
bitcoin-qt balances display as expected, before and after sending a tx

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2401