Remove redundant information about Minitest, which is already covered when
assertions are introduced. [ci skip]
Condense notes regarding Minitest and its available assertions removed in:
82b75eda70f8c30f7c418e7350532a3a0ed2a8c3

Also explain how Rails inherits capabilities from Minitest. Test classes should
be roughly the same based on this information, we can address them at a high
level when introducing the first test case. [ci skip]