Reorganization of the Unit Testing Lecture

[john-science]

Currently, we just have one unit testing lecture on the syllabus/home page:

• Unit Tests - lecture_17.md

But I think it would be better if we broke this up into a few lectures:

• Unit Tests - lecture_17.md
  • PyTest #81 - lecture_17.2.md
  • Good Tests and Testable Code - lecture_17.4.md
    • (mocking and code coverage?)