Correct volume mount point

Description

During development work on bespoke benchmarking it became clear that the volume construction in the default benchmark did not match what it looks like was intended this would seem to be due to an assumption of what the relative path is when running the script in the volume construction. Also it seems that mounting via a pre-existing directory '/src' meant that the mount point did not actually appear.

Changes proposed in this merge request:

• Change from using '/src' to '/mnt/baserock-source' which does not pre-exist in the docker image
• Put in check to make sure that the volume is mounted in the main body of the test.
• Use absolute paths when checking out the baserock definitions.