Egg building tests leave 'build' directories behind

[mdickinson]

Some of our tests dynamically build eggs. With current versions of setuptools and pip, that results in build directories being left behind in the source tree. We should be cleaning those build directories up. (The lack of cleanup is currently hidden by our .gitignore; we may want to fix this.)

[mdickinson]

We should also clean up the egg-info directories created.

[mdickinson]

Let's punt on this - it's not causing any real issues. We're already excluding build and egg-info directories in .gitignore, and all of the egg building machinery should be going away in Envisage 8.0 anyway. (xref: #548)

For the record, this was causing issues at one point during development because the built eggs were picking up the build directories as part of the package, leading to a several-layers-deep recursive inclusion of build directories. #543 fixed this particular issue by introducing specific inclusions in the find_packages calls in setup scripts.