client/web/src/regression: update regression test docs

Created by: slimsag

This updates the regression test docs to clarify how to run these tests - these docs were really outdated / broken, I had to do a fair amount of digging.

I added a 1password note with the .envrc file I got from Uwe, because you need to set like ~15 env vars in order to run these tests.

I also removed the comments about wanting customers in the future to run this test suite - I don't think we want that anymore and that experience would not be good as running this test suite requires a working Sourcegraph dev env. The src validate command Uwe added to src-cli seems a lot better from a customer POV: https://github.com/sourcegraph/src-cli/pull/200