Test installation of very large lazily-synced package

Original Pulp Redmine Issue: https://pulp.plan.io/issues/7402

Some of the largest packages, such as TensorFlow, can be as large as 340 megabytes in size [0]. Some may even be larger.

If such packages are lazily-synced, and a user tries to install them from Pulp, we should ensure that any delays incurred by Pulp in the process of downloading these packages in the background does not interfere with the operation of the client making the request to Pulp.

A manual test would be acceptable, but an automated test that is not part of the standard CI suite would be better.

[0] https://pypi.org/project/tensorflow/2.3.0/#files

No source file or test entry point is named. Start by locating the Pulp Python plugin's existing lazy-sync and installation tests, then exercise a TensorFlow-sized package from PyPI while it downloads in the background. Done means confirming that the client installation request is not disrupted; a manual test or an automated test outside standard CI is acceptable.