The test.summary rule was causing errors in our Gitlab testing due to
multiple runs (concurrent or otherwise) in the same workspace directory.
This patch removes the WORKSPACE directory variable, and each .testing
run happens in its own directory.

Other minor changes:

- The script to generate the summary was moved out of the Makefile and
  into a separate script.

- Unrelated to these changes, error output was extended from 20 to 40
  lines, to provide more readable backtrace output.