Segregate uploads during parallel test runs

Avoids parallel processes stomping on each other's uploads during
test runs. The nice side-effect of this is that we no longer need the
special-case cucumber tagging for testing virus checking.
Bulk upload behaves consistently during tests

Rather than monkey patching the default root directory for bulk
uploads in multiple places, we can set it according to the env during
initialisation. For consistency with attachment uploads, it's now
in tmp/test.