Upgrade (currently ignored) regTests to work with Bitcoin Core 30+

[msgilligan]

This is required before we can do a nix flake update to update the Bitcoin Core version used for RegTest to Bitcoin Core 30.0 or later. (I disabled those tests and went ahead with the nix flake update)

Bitcoin Core 30.0 removes support for "BDB legacy wallets" which some of our RegTest Integration Tests rely on. The quick solution is to just add a flag to disable these tests. Longer term we need to port the "regtest mining" infrastructure to work with descriptor wallets. See the description of PR #209 for details.

PR #228 was the PR that disabled (ignored) the tests that require BDB wallets.

[msgilligan]

The following PR re-enables most tests to work with Bitcoin Core v30.2 as well as making them work with Omni Core v0.12.0 / Bitcoin Core v20.1.

• PR Re-enable most regTests, re-enable Omni Core / Bitcoin Core v20.1 support #350

It turns out that two minor fixes (the first two commits on PR 350) fix most of the disabled tests. The only known remaining issue is that modern Bitcoin Core doesn't support privkeydump, so a new flag has been added in gradle.properties that can be used to disable the tests that rely on the privkeydump JSON-RPC method.