tests/functional: factor out common code in gpu test

tests/functional: factor out common code in gpu test

In preparation for handling more tests split out the common machine
setup details from the test specific stuff and add a helper for
launching the weston test. Instead of searching for "vkmark score" we
set a custom PS1 and wait for a successful completion. This ensures we
capture the score in the console log which otherwise wouldn't log
anything.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250304222439.2035603-3-alex.bennee@linaro.org>