Use the Golang elasticsearch indexer in tests

What does this MR do?

In preparation for the removal of the Ruby elasticsearch indexer, this MR adds the Golang indexer to the test environment, and ensures all tests are run using it.

Does this MR meet the acceptance criteria?

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

Test setup is slowed down a bit (20 seconds), but that's unavoidable. The setup happens just once, as part of TestEnv.init, and the compiled binary is copied to all test jobs, so this should be relatively cheap, compared to compiling it once per job.

The binary is 22MiB, so the gitlab-ee artifact increases by that much.

Related to #6481 (closed)