test: mark test-http-regr-nodejsgh-2928 as flaky

It has been flaky for more than a year. Mark it as flaky to avoid
blocking the CI.

PR-URL: nodejs#49565
Refs: nodejs/reliability#658
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Ruy Adorno <ruyadorno@google.com>

test/sequential/sequential.status

@@ -15,6 +15,10 @@ test-watch-mode-inspect: PASS, FLAKY
 # https://github.com/nodejs/node/issues/47116
 test-http-max-sockets: PASS, FLAKY
 
+[$system==win32 || $system==freebsd]
+# https://github.com/nodejs/node/issues/49564
+test-http-regr-gh-2928: PASS, FLAKY
+
 [$system==linux]
 
 [$system==macos]