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 <mdawson@devrus.com>

PR-URL: https://github.com/nodejs/node/pull/41203
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Adrian Estrada <edsadr@gmail.com>

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]