Document testing best practices

[mcomella]

• Test naming conventions
• UI vs integration vs unit: when to write which?
• androidTest package hierarchy
• robot pattern

[mcomella]

* Test naming conventions

* UI vs integration vs unit: when to write which?

* androidTest package hierarchy

* robot pattern

Landed all of these in https://github.com/mozilla-mobile/shared-docs/blob/master/android/testing.md

Still TODO:

• How we override the application class in each case
• How we do ServiceLocator injections
• ...?