Add infra necessary to run API tests with specific server config

[acelaya]

Part of #1009

Add necessary configuration to run API tests with a different server config, and one test using it as a proof of concept.

The process works as follows:

1. Tests with the #[WithEnvVars([...])] attribute will cause the RoadRunner server to be restarted including the list of env vars.
2. Once the test has been run, the server will be restarted again without those env vars.

For this to work, this PR defines a PHPUnit extension that registers two listeners.

• One listener runs before tests, checking if the attribute is set. If so, it creates a temp env vars file, and restarts the RoadRunner server.
• The other listener runs after every test. If the env vars temp file exists, it deletes it and restarts the server again, so that following tests do not run with the env vars from previous test.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.89%. Comparing base (1c38ab1) to head (8892407).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop    #2468      +/-   ##
=============================================
+ Coverage      93.61%   95.89%   +2.27%     
  Complexity      1760     1760              
=============================================
  Files            279      279              
  Lines           6012     6012              
=============================================
+ Hits            5628     5765     +137     
+ Misses           384      247     -137     

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[acelaya]

These changes seem to be working consistently in CI, but in my local dev env the tests are very flaky, or they even make other tests fail even if the env-specific tests pass.

I'm still not sure what's the reason. It feels like something is "cached" and the wrong env vars are loaded, or perhaps it's a timing issue, although I have tried adding some sleep calls and it doesn't seem to help.

Another thing that could be causing this is that an env var is set, and then reloading the server, even if the env vars file is no longer present, does not reset previously "promoted" env vars simply because the process is still running. I need to verify if this is the case.