Remove the kyua-test-filters(7) manpage

This manual page is auxiliary and very short, so it is distracting to keep
around.  Fold the contents into the pages that need a description of test
files, and do so using our shiny new "file inclusion" functionality in
manbuild.
Remove the kyua-build-root(7) manpage

Fold the contents of kyua-build-root(7) into all the pages that need a
description of build directories.  This makes the contents more clear
as they can refer to the calling command instead of being "generic" and
talking about kyua(1) as a whole.
Describe --results-file in each command

Move the "abstract" description of the --results-file flag supported by
various commands from the kyua-results-files(7) manpage to the manpage
of every command that supports the flag.