Full test coverage

[chuckwondo]

Increase test coverage to 100%, plus a bit of further tidying:

• Use python -m pip instead of pip directly in Makefile to ensure correct installation of pip is used
• Upgrade pip in CI to eliminate "newer pip available" message
• Uniquely name test case that was a duplicate name (likely an earlier copy-paste error)

[chuckwondo]

@francium, before I submit a PR for #9, I thought I would do a bit of tidying of test coverage to keep my PRs a bit more granular for you.

[francium]

👍