improve handling of zipped test data

[atravitz]

the cdk_8 dataset gets unzipped during testing, but creates files in the user's workspace that we do not want to be committed into the repo. I propose we unzip test data into a tmp directory during testing, so that clean up is handled by pytest.

[atravitz]

looks like the zip file is 216K, and the unzipped data is 864K. @mikemhenry do you have a rule of thumb for max file sizes? 864K might be fine based on what we were discussing here?

[mikemhenry]

Given our current file and directory sizes ~1MB seems like a nice value to target so we can leave it unzipped