Expand coverage for pip install test

Start by locating the existing pip install test for shelf-reader and review the pip documentation on selecting sdist and wheel packages. The work is complete when the test covers installation from both package types and verifies the expected installation behavior.

Original Pulp Redmine Issue: https://pulp.plan.io/issues/6838

The pip install test uses only the sdist (.tar.gz) package type when checking that it can install 'shelf-reader'. However, the repository has an additional wheel (.whl) version that could be installed from but isn't checked.

Currently the default behavior for pip install (Verision 20.1.1 on Python 3.7) seems to use the sdist type if the package 'wheel' is not installed (see pip install message below). Checking the pip forums the discussion on the workings of pip install seem to be ongoing and could be subject to change.

When documentation is available for the workings of pip install the pip install test should be expanded to test installing from different package types.

$ pip install --trusted-host localhost -i http://localhost:24816/pulp/content/4352fcae-4ca3-4df1-ab0f-199adc3cec8c/simple/ shelf-reader
Looking in indexes: http://localhost:24816/pulp/content/4352fcae-4ca3-4df1-ab0f-199adc3cec8c/simple/
Collecting shelf-reader
  Downloading http://localhost:24816/pulp/content/4352fcae-4ca3-4df1-ab0f-199adc3cec8c/shelf-reader-0.1.tar.gz (19 kB)
Using legacy setup.py install for shelf-reader, since package 'wheel' is not installed.
Installing collected packages: shelf-reader
    Running setup.py install for shelf-reader: started
    Running setup.py install for shelf-reader: finished with status 'done'
Successfully installed shelf-reader-0.1