test: remove flaky designations for tests

Three tests designated as flaky on Linux have not failed on the
continuous integration server in a long time. Removing flaky designation
for these tests.

Fixes: https://github.com/nodejs/node/issues/4446
PR-URL: https://github.com/nodejs/node/pull/4519
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>

test/parallel/parallel.status

@@ -13,9 +13,6 @@ test-cluster-shared-leak             : PASS,FLAKY
 test-tls-ticket-cluster              : PASS,FLAKY
 
 [$system==linux]
-test-http-client-timeout-event       : PASS,FLAKY
-test-child-process-buffering         : PASS,FLAKY
-test-child-process-exit-code         : PASS,FLAKY
 
 [$system==macos]