test: mark flaky tests on Raspberry Pi

A few tests have started failing on Raspberry Pi devices in CI.
https://ci.nodejs.org/job/node-test-binary-arm/943/

PR-URL: https://github.com/nodejs/node/pull/5082
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Ref: https://github.com/nodejs/node/issues/4830
Ref: https://github.com/nodejs/node/issues/3635
Ref: https://github.com/nodejs/node/issues/4526

test/parallel/parallel.status

@@ -13,6 +13,7 @@ test-tick-processor                  : PASS,FLAKY
 
 [$system==linux]
 test-tick-processor                  : PASS,FLAKY
+test-child-process-fork-regr-gh-2847 : PASS,FLAKY
 
 [$system==macos]
 

test/sequential/sequential.status

@@ -10,6 +10,8 @@ prefix sequential
 
 [$system==linux]
 test-vm-syntax-error-stderr : PASS,FLAKY
+test-dgram-pingpong         : PASS,FLAKY
+test-http-regr-gh-2928      : PASS,FLAKY
 
 [$system==macos]