Pass -noreset by default to Xvfb to avoid a common gotcha.

X server has a notion of "generations" (described in https://stackoverflow.com/questions/8340314/xorg-stops-working-after-all-x-applications-has-exited/8359526#8359526) which means it performs a "reset" (i.e. a re-initialization) each time all X clients disconnect.

For a short while, while this reset is in progress, the X server does not accept new connections. This causes intermittent connection issues and therefore flakes in tests which use Xvfb in their harness. Less powerful architectures are especially prone to this problem.

This is a rather obscure feature, and catches many people by surprise. There is evidence on the internet (see https://bugs.debian.org/981201 for an example) of people adding "-noreset" as a workaround in their test scripts.

IMHO adding this here instead is a better way forward than the alternative of multiple developers learning about this the hard way. Users who do care about the "reset" feature can still trivially override the default with "--server-args".