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### What
We've had our CI mixed between Github and Buildkite for a while, it's
time to commit. First step is moving the "tests" step to Github Actions.
<!-- What is this PR trying to accomplish (and why, if it's not
obvious)? -->
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### How
This PR:
- Moves the `test` step to Github Actions
- Creates a new `custom_connector.Dockerfile` which builds custom
connector only, more quickly.
- Changes the metadata tests to use `localhost` instead of their Docker
internal names (ie `custom_connector` or `postgres_connector`) - this is
because the tests are being run from outside Docker now
- Removes the `test` Buildkite step
It does not:
- Remove the code coverage or benchmarks steps from Buildkite
- Tidy up `justfile` or Dockerfiles
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steps)? -->
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Co-authored-by: Philip Lykke Carlsen <plcplc@gmail.com>
V3_GIT_ORIGIN_REV_ID: a67534ebc1634a24b48d2620c45003221852e199
