Can't run test-patch with start-build-env.sh

The Docker instance started by start-build-env.sh drops the user into a root directory wherein other directories (say ~/download) are not accessible, so one cannot pull patches to test into it. The dev-support/test-patch.sh script requires a clean environment with no extra files, such as the patch to test. Between these two restrictions, one can't use the docker env to run test-patch. We should either -v a volume where patches can be stashed, or allow test-patch.sh to ignore patch files' existences.