tests: refactor systests into separate modules w/ pytest fixtures (#393)

* tests: break out shared setup / teardown into pytest fixtures

* tests: move instance admin systests into a separate module

* tests: move table admin systests into a separate module

* tests: move data API systests into a separate module

* tests: remove remaining monolith fixtures

Closes #391.

* test: harden flaky 'w_two_clusters' systest against GRPC Unavailable

Ported from PR #385.

Fixes #381.