Improve ConfusionMatrix GetSummary formatting, and several tiny improvements

Tiny improvements include:

- fix typo
- go fmt ./...
- remove several unused untested, often private functions
- remove redundant import renames, e.g. `pkgname "github.com/user/repo/pkgname"`
- favor idiomatic error returns instead of panics
- favor idiomatic `testing.T` failure functions instead of panics
- remove unnecessary `Println`s from tests (automated tests should make assertions, not just print to STDOUT)
- consistently use `(t *testing.T)` instead of sometimes `(testEnv *testing.T)` or `(T *testing.T)`