Make test_download_warcs work without changing directory

[amhanson9]

Currently, test_script.py does not work when running all the tests in the tests folder because it relies on the current directory being the tests directory and test_download_warcs is changing the current directory in setUp and tearDown. I can't remember why changing it in setUp was necessary. That, in turned, made it necessary to change for tearDown to be able to delete the folder created during the test.

The current directory is also changed in test_unzip_warc, but since it runs after test_script.py, it isn't impacting anything. The rest of the tests are not dependent on what the current working directory is.

The work around until this is fixed is to run all the tests in the tests folder (ignoring the errors from test_script.py) and then run test_script.py separately.