New testing procedure for x86 and x86_64

[mhsmith]

I no longer have a desktop x86_64 machine, so we need a new way of running the unit tests (both Java and Python) and package tests on these architectures. Something that can be run interactively at the Linux command line, similar to the CPython testbed.

Consider whether we should include it in GitHub Actions, which would save some time in future releases, and give some early warning of problems, as we've already seen with the integration tests. We'll need to include all Python versions, but one Android version is enough, as we'll already be testing the full range on ARM64.