unittest: Disable racy test

When run on a slow/busy machine, the notification might happen in 2 or
more batches. It works most of the time, but it's causing the build to
fail intermittently.
Status Job ID Name Coverage
  Test
passed #279138
amd64 debian
asan-ubsan

00:10:49

passed #279135
amd64 debian
linux

00:09:30

passed #279136
win32
win32

00:11:17

passed #279137
win64
win64

00:08:38