remotewrite/sender: Fixed RW sending compliance tests

[bwplotka]

During our LFX mentorship, @pipiland2612 redesigned our compliance sending tests -- in-line with #182. The #194 PR was pending for long time and unfortunately I merged it without careful review to iterate.

The introduced refactor added a lot of good RW2 tests, but had a lot of inconsistencies, false positive, noop and duplicated tests, (e..g exemplars passing, without detecting critical issue). I should have review previous PR carefully and guide @pipiland2612 , but I didn't, so trying to fix it here.

The work was MUCH easier with @pipiland2612 initial PR, I just had to clean up and adjust some tests. I left out some commented code, feel free @pipiland2612 to help and add PR to this PR to iterate 🤗

Changes:

• Added makefile for easier use
• Allow arbitrary process execution as target (this allows setting CI on prometheus/prometheus easier).
• Moved all tests into a single scrape->target->receiver sendertest framework. No exception.
• Removed tons of tests that didn't make sense (we cannot reliably test them, dups, not true to the spec)
  • TODO: List all once done and why
• Tighten up tests
• Moved all semi-RW spec related tests to prometheus_test file and ensured RECOMMENDED for those.
• Modernized CI job.
• Added preloading feature to HTML for easier use (avoiding CORS issue)

TODO:

• Clean up CI
• Add CI on prometheus/prometheus to prove

This cleans and fixes up the early test we merged recently

[bwplotka]

Talking internally with @pipiland2612 and @saswatamcode on how to split/review/finish this

[bwplotka]

Moving to a feature branch as agreed #208