Use z3 for CI

My CI end-to-end tests use zcashd/lightwalletd in regtest mode. The test goes something like this:
- deploy a new chain in regtest
- generate 100 blocks
- shield
- move some funds to an address wallet
- perform some payments
- check tx history and balances

Ref: https://github.com/hhanh00/zcash-walletd/actions/workflows/build-macos.yaml

It'd good to be able to do the same thing with z3.
