test: reduce http-server-request-timeouts-mixed flakiness

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

This change makes the checking interval more frequent, and decouples
the timeout for the check from the checking interval.

PR-URL: #44169
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-http-server-request-timeouts-mixed.js

@@ -15,7 +15,7 @@ const responseOk = 'HTTP/1.1 200 OK\r\n';
 const responseTimeout = 'HTTP/1.1 408 Request Timeout\r\n';
 
 const headersTimeout = common.platformTimeout(2000);
-const connectionsCheckingInterval = headersTimeout / 4;
+const connectionsCheckingInterval = headersTimeout / 8;
 
 const server = createServer({
   headersTimeout,
@@ -76,7 +76,8 @@ server.listen(0, common.mustCall(() => {
 
     // Send the second request, stop in the middle of the headers
     request2.client.write(requestBodyPart1);
-    // Send the second request, stop in the middle of the headers
+
+    // Send the third request and stop in the middle of the headers
     request3.client.write(requestBodyPart1);
   }, headersTimeout * 0.2);
 
@@ -111,7 +112,7 @@ server.listen(0, common.mustCall(() => {
 
     assert(request1.response.startsWith(responseOk));
     assert(request2.response.startsWith(responseTimeout)); // It is expired due to headersTimeout
-  }, headersTimeout * 1.2 + connectionsCheckingInterval);
+  }, headersTimeout * 1.4);
 
   setTimeout(() => {
     // Complete the body for the fourth request