E2E test fixes

[SteveSandersonMS]

As of this PR:

• The Ignitor tests are all skipped.
  • Previously, they were already quarantined, but still ran and failed locally. As previously discussed on the team, we should remove both the Ignitor tests and Ignitor itself, but that can be done in a later PR.
• Every other E2E test now passes, at least if you re-run enough times
  • Previously, there were a number of quarantined tests that would never pass due to bugs in either the test implementation or (in several cases) the recent string interpolation bug in .NET Core itself. These are all now fixed/workarounded, so I've seen them all go green at least once.

This doesn't address the reliability/consistency concerns, which we still need to take on as a later piece of work. Running the tests locally I don't see many (or any?) random failures of the form "expected x, got y" but I do see random failures like "the test server did not start". This might be assuaged by running without parallelism but it's a whole separate investigation.

[javiercn]

Looks great, thanks for taking on this.

[SteveSandersonMS]

Running locally now:

This includes running the quarantined tests. The skipped ones are the ignitor ones as described above.

[SteveSandersonMS]

Thanks for approving, @javiercn and @TanayParikh. I was a bit premature in requesting review on this PR, since I needed to make a bunch of other small changes before it was ready. If either of you wants to review the changes since you saw it, please go ahead, but no need to if you don't feel like it because the changes are pretty minor, test-only, and I think noncontroversial.

[SteveSandersonMS]

Also just for the record, I've run the E2E tests 5 times in a row locally this afternoon, and all 5 times got a 100% pass rate.

I'm going to keep running these tests on a loop to collect more data, but at this point it looks like remaining reliability issues are probably CI-specific and not inherent to these tests.

[SteveSandersonMS]

/backport to release/6.0-rc1

[github-actions]

Started backporting to release/6.0-rc1: https://github.com/dotnet/aspnetcore/actions/runs/1143942181