LSIF: Refactor tests

Created by: efritz

Refactor tests around the backend. This moves the test files that act as more of an integration test (creating temporary postgres dbs, uploading files, checking results through backend methods) into a subdirectory with some common utils (setup, teardown of storage root and temp databases).

This also structures the test data generation in a more formalized way.

This is a prerequisite to work done on RFC 58, which requires adding another such test (which copies a lot of boiler plate and generates very similar test data). It was hectic to keep doing this in an ad-hoc way.