test: mark test-http-server-request-timeouts-mixed as flaky

This has been flaking the CI for more than 2 years with various
attempts to fix without success. It has still been flaking the
CI (failed 19 out of 100 recent testing CI runs). It's time to
mark it as flaky.

test/sequential/sequential.status

@@ -28,6 +28,9 @@ test-http-server-request-timeouts-mixed: PASS, FLAKY
 # https://github.com/nodejs/node/issues/54816
 test-single-executable-application-empty: PASS, FLAKY
 
+# https://github.com/nodejs/node/issues/43465
+test-http-server-request-timeouts-mixed: PASS, FLAKY
+
 [$system==solaris] # Also applies to SmartOS
 
 [$system==freebsd]