Move conceptual test info into "Getting Started with Testing" and

move the procedural information into the corresponding guides for creating
unit tests.

This is part of a cleanup for the Android Studio test docs, simplifying the
amount of content covered with Android Studio to be strictly how to create
and run tests. Bug: 28470079
Related: https://critique.corp.google.com/#review/125115510

Change-Id: I8b9590045b42a03c618e7011e0c3741aede77cd8