Use testcontainers for DB tests

[lachiewalker]

Adds testcontainers to:

• automate DB stand-up and tear-down for unit tests
• standardise DB config (TZ=UTC required to pass test suite now configured in conftest)
• Enable automated testing of edge case and optional features such as custom tablespaces (add tablespace argument to Table #527)

conftest now orchestrates DB rather than the github CI workflow.