test: mark connection leak test flaky on IBM i

Mark `test-http-pipeline-requests-connection-leak` flaky on IBM i.

PR-URL: #44215
Refs: #43509
Refs: #39683
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

test/parallel/parallel.status

@@ -84,6 +84,7 @@ test-crypto-secure-heap: SKIP
 test-dgram-connect: PASS, FLAKY
 test-http-client-parse-error: PASS, FLAKY
 test-http-multi-line-headers: PASS, FLAKY
+test-http-pipeline-requests-connection-leak: PASS, FLAKY
 test-http-server-unconsume: PASS, FLAKY
 test-http-upgrade-advertise: PASS, FLAKY
 test-tls-client-mindhsize: PASS, FLAKY