test: Don't hardcode the port to listen on

Hardcoding the port to let the test server listen on is can be
problematic in case the build hosts happens to have something on that
port (e.g. another daemon or build running at the same time). Instead
let the kernel pick a free port for us.

Apart from that, this changes all configuration files to be
templated, with HOST expected to be updated with the actual address/port
picked. For explicit configuration files there is a new context manager
which will provide a temporary config file with the right information
while the sysroot http server will setup a temporary directory to
serve files from (with all files updates with correct url information
as well).

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5243