Make a list of each test suite's default tests and extra tests in README

[MonicaisHer]

A list can help us quickly go through which tests are disabled by default.

For example, in edgexfoundry suite, the refresh test is disabled for a local build snap by default. When a PR is related to upgrading, we need to upload that local build snap to snapstore first, and then manually run the refresh test locally.

[farshidtz]

Describing each test suite and what it does is good, but maintaining the configuration documentation will be hard. Instead, I think we should modularize tests so we don't have to remember or document conditional testing workflows.

See also:

• Make refresh tests work with local builds #131
• Separate refresh test suites #117