parallel/http-server-request-timeouts-mixed test was sometimes failing due to insufficient tolerance between the connection timeout checking interval, and the expected timeout specified in the test.

The checking interval was 500ms, and the request was checked for timeout exactly 500ms after the request was expected to timeout. This led to a timing condition where the next check would occur slightly after the request was expected to timeout.

Detailed Explanation
Timings
headersTimeout: 2000ms
requestTimeout: 4000ms
connectionsCheckingInterval: 500ms

Request 2 is started at headersTimeout * 0.2, or 400ms.
Headers time out 2000ms after that.
Completion of Request 2 is checked at at headersTimeout * 1.2 + connectionsCheckingInterval, or 2900ms.

Timeline
@[400ms]: Request 2 client created and first write sent.
@[2400ms]: Headers are timed out.
@[2900ms]: Request 2 completion is checked (which is intended to depend on Request 2 expiring via headersTimeout, as seen in the comment - assert(request2.response.startsWith(responseTimeout)); // It is expired due to headersTimeout.

The problem is that the connectionsCheckingInterval is slightly too high. The connection checking interval could tick just after 2400ms, and then just after 2900ms.

Example

@[2424ms] request2.completed: false
checking connection timeout
@[2525ms] request2.completed: false
@[2625ms] request2.completed: false
@[2727ms] request2.completed: false
@[2828ms] request2.completed: false
node:assert:400
    throw err;
    ^

AssertionError [ERR_ASSERTION]: The expression evaluated to a falsy value:

  assert(request2.completed)

    at Timeout._onTimeout (/Users/nicksia/Documents/git/node/test/parallel/test-http-server-request-timeouts-mixed.js:108:5)
    at listOnTimeout (node:internal/timers:564:17)
    at process.processTimers (node:internal/timers:507:7) {
  generatedMessage: true,
  code: 'ERR_ASSERTION',
  actual: false,
  expected: true,
  operator: '=='
}

The fix is to ensure connection checking interval is less than 500ms, and to decouple the check timeout from connection checking interval.