Timeout in config.ta_bootstrap test on Meson 0.57.0 onwards

It seems that on Meson 0.57.0 onwards the config.ta_bootstrap test fails with a timeout because the webserv.lua process does not terminate. If stdout and stderr are redirected to /dev/null this problem does not occur.

I think this is a regression in Meson, but since I doubt there will be a backport for the version of Meson included in the current distros and the workaround is simple, it is worth fixing it here.

I encountered this problem when trying to build Knot Resolver on Ubuntu 22.04.