Add testing.md

Signed-off-by: Daniel Nephin <dnephin@docker.com>

TESTING.md

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+# Testing
+
+This document contains the Moby code testing guidelines. It should answer any 
+questions you may have as an aspiring Moby contributor.
+
+## Test suites
+
+Moby has two test suites (and one legacy test suite):
+
+* Unit tests - use standard `go test` and
+  [testify](https://github.com/stretchr/testify) assertions. They are located in
+  the package they test. Unit tests should be fast and test only their own 
+  package.
+* API integration tests - use standard `go test` and
+  [testify](https://github.com/stretchr/testify) assertions. They are located in
+  `./integration/<component>` directories, where `component` is: container,
+  image, volume, etc. These tests perform HTTP requests to an API endpoint and
+  check the HTTP response and daemon state after the call.
+
+The legacy test suite `integration-cli/` is deprecated. No new tests will be 
+added to this suite. Any tests in this suite which require updates should be 
+ported to either the unit test suite or the new API integration test suite.
+
+## Writing new tests
+
+Most code changes will fall into one of the following categories.
+
+### Writing tests for new features
+
+New code should be covered by unit tests. If the code is difficult to test with
+a unit tests then that is a good sign that it should be refactored to make it
+easier to reuse and maintain. Consider accepting unexported interfaces instead
+of structs so that fakes can be provided for dependencies.
+
+If the new feature includes a completely new API endpoint then a new API 
+integration test should be added to cover the success case of that endpoint.
+
+If the new feature does not include a completely new API endpoint consider 
+adding the new API fields to the existing test for that endpoint. A new 
+integration test should **not** be added for every new API field or API error 
+case. Error cases should be handled by unit tests.
+
+### Writing tests for bug fixes
+
+Bugs fixes should include a unit test case which exercises the bug.
+
+A bug fix may also include new assertions in an existing integration tests for the
+API endpoint.
+
+## Running tests
+
+To run the unit test suite:
+
+```
+make test-unit
+```
+
+or `hack/test/unit` from inside a `BINDDIR=. make shell` container or properly
+configured environment.
+
+The following environment variables may be used to run a subset of tests:
+
+* `TESTDIRS` - paths to directories to be tested, defaults to `./...`
+* `TESTFLAGS` - flags passed to `go test`, to run tests which match a pattern
+  use `TESTFLAGS="-test.run TestNameOrPrefix"`
+
+To run the integration test suite:
+
+```
+make test-integration
+```