Tests: add HTTP robust retries test

[sbhowmikf5]

Test retries after all upstream peers are exhausted with the "no_live" parameter, including retry limits and delayed retries controlled by "proxy_next_upstream_delay".

Issue - nginx/nginx#1263
Feature PR - nginx/nginx#1757

Checklist

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• I have read the [contributing guidelines.
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• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.

[route443]

Hi @sbhowmikf5 ,
I slightly changed the approach and re-write your tests, adding you as a co-author:

1. Split the changes into two files, since a new proxy_next_upstream_delay direcive was introduced.
2. I added gating based on the config, though this will affect the proxy_next_upstream.t as well. I think that's acceptable.
3. Since we are dealing with delays, splitting the tests into separate files will have a positive impact on CI runtime.
4. Added comments for the conf as well
5. Reviewed code coverage and expanded the tests (72% -> 91% by gcov)
6. Right now (against commits 466f7ba and 5bf8a71), the tests are failing, which is expected. The issues need to be addressed, and in theory, the tests should then pass :)