tests: Run `test-empty-sync` in GCP to test the "no cached state" workflow

[teor2345]

Describe the issue or request

If we run the test-empty-sync test inside GCP on every PR, it will test the workflow that's used for the long scheduled jobs.

The test is currently run on a GitHub runner:

zebra/.github/workflows/continous-integration-docker.yml

Lines 226 to 245 in 004e436

	# Test that Zebra syncs and checkpoints a few thousand blocks from an empty state.
	#
	# (We activate the gRPC feature to avoid recompiling `zebrad`, but we don't actually run any gRPC tests.)
	test-empty-sync:
	name: Test checkpoint sync from empty state
	runs-on: ubuntu-latest
	needs: build
	if: ${{ github.event.inputs.regenerate-disks != 'true' && github.event.inputs.run-full-sync != 'true' && github.event.inputs.run-lwd-sync != 'true' }}
	steps:
	- uses: r7kamura/rust-problem-matchers@v1.4.0
	- name: Inject slug/short variables
	uses: rlespinasse/github-slug-action@v4
	with:
	short-length: 7
	- name: Run zebrad large sync tests
	run: |
	docker pull ${{ vars.GAR_BASE }}/${{ vars.CI_IMAGE_NAME }}:sha-${{ env.GITHUB_SHA_SHORT }}
	docker run -e NETWORK --name zebrad-tests -t ${{ vars.GAR_BASE }}/${{ vars.CI_IMAGE_NAME }}:sha-${{ env.GITHUB_SHA_SHORT }} cargo test --locked --release --features lightwalletd-grpc-tests --package zebrad --test acceptance -- --nocapture --include-ignored sync_large_checkpoints_

We might need to use "--test-threads 1" for this test, but it seems to work without it.

Additional Information/Context

Optional: test-lightwalletd-integration is another alternative if we want to test empty Zebra and lightwalletd states.