test: mark test-http-agent flaky

Ref: https://github.com/nodejs/node/issues/5184
PR-URL: https://github.com/nodejs/node/pull/5209
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>

test/parallel/parallel.status

@@ -7,12 +7,13 @@ prefix parallel
 [true] # This section applies to all platforms
 
 [$system==win32]
-test-debug-no-context                : PASS,FLAKY
-test-tls-ticket-cluster              : PASS,FLAKY
-test-tick-processor                  : PASS,FLAKY
+test-debug-no-context   : PASS,FLAKY
+test-tls-ticket-cluster : PASS,FLAKY
+test-tick-processor     : PASS,FLAKY
 
 [$system==linux]
-test-tick-processor                  : PASS,FLAKY
+test-http-agent         : PASS,FLAKY
+test-tick-processor     : PASS,FLAKY
 
 [$system==macos]