util/expvarx: deflake TestSafeFuncHappyPath with synctest

I probably could've deflaked this without synctest, but might as well use
it now that Go 1.25 has it.

Fixes #15348

Change-Id: I81c9253fcb7eada079f3e943ab5f1e29ba8e8e31
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f