Create Testing Strategy docs

[fredvisser]

Copied from User Story 1594615: Create Testing Strategy docs

We should provide an overview of the testing architecture of nimble and its dependencies

This could go in each package's CONTRIBUTING.md or a file linked from it. We already have some docs for nimble-angular but need some for nimble-components that cover: unit testing vs storybook testing when to rely on FAST tests vs write our own links to test utilities and descriptions of test patterns

[jattasNI]

Assigned to myself to evaluate whether what we have is good enough