tests/functional: add helpers for building file paths

tests/functional: add helpers for building file paths

Add helper methods that construct paths for

 * log files - to be preserved at the end of a test
 * scratch files - to be purged at the end of a test
 * build files - anything relative to the build root
 * data files - anything relative to the functional test source root
 * socket files - a short temporary dir to avoid UNIX socket limits

These are to be used instead of direct access to the self.workdir,
or self.logdir variables, or any other place where paths are built
manually.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20241217155953.3950506-11-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>