test: mark child-process-exec-kill-throws flaky

PR-URL: https://github.com/nodejs/node/pull/12054
Ref: https://github.com/nodejs/node/issues/12053
Ref: https://github.com/nodejs/node/pull/11038
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

test/parallel/parallel.status

@@ -7,6 +7,9 @@ prefix parallel
 [true] # This section applies to all platforms
 
 [$system==win32]
+# See https://github.com/nodejs/node/issues/12053, this test may be exposing a
+# genuine bug with kill() on Windows.
+test-child-process-exec-kill-throws : PASS,FLAKY
 
 [$system==linux]