Bisq apitest docs

build-run.md: Build and run API tests at the command line and from Intellij.
test-categories.md: How to categorize a test case as method, scenario or e2e.
regtest-port-conflicts.md: Avoid port conflicts when running multiple bitcoin-core apps in regtest mode.
api-beta-test-guide.md: How to run the test harness and tutorial script, and beta test the API with the new CLI.