test: mark wpt/test-user-timing test flaky

- The RH team had a team day looking at helping move
  the CI closer to green. This is one of the flaky tests
  with the most reported failures and has been open
  since October. Marking flaky.

Signed-off-by: Michael Dawson <[email protected]>

PR-URL: #41203
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Adrian Estrada <[email protected]>

Author: mhdawson

test/wpt/wpt.status

@@ -5,6 +5,8 @@ prefix wpt
 # sample-test                        : PASS,FLAKY
 
 [true] # This section applies to all platforms
+# https://github.com/nodejs/node/issues/40449
+test-user-timing: PASS,FLAKY
 
 [$system==win32]