test: set benchmark-child-process flaky on windows

sequential/test-benchmark-child-process is still failing sometimes flaky
on Windows in CI. Mark it as flaky in sequential.status until it gets
sorted.

PR-URL: #12561
Ref: #12560
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

test/sequential/sequential.status

@@ -7,6 +7,7 @@ prefix sequential
 [true] # This section applies to all platforms
 
 [$system==win32]
+test-benchmark-child-process  : PASS,FLAKY
 
 [$system==linux]