docs: add documentation on how to run tests (#2288)

Fixes #300

Author: chingor13

README.md

@@ -34,9 +34,14 @@ there are no such images published at the moment.
 See https://pkg.go.dev/github.com/googleapis/librarian/cmd/librarian for
 additional documentation.
 
+## Running Tests
+
+See [testing.md] for more information.
+
 ## License
 
 Apache 2.0 - See [LICENSE] for more information.
 
 [contributing]: CONTRIBUTING.md
 [license]: LICENSE
+[testing.md]: doc/testing.md

doc/testing.md

@@ -0,0 +1,49 @@
+# Testing
+
+## Unit Tests
+
+These tests are designed to test the internal librarian golang logic.
+
+Usage:
+
+```bash
+go test ./...
+```
+
+## End-to-End (e2e) Tests
+
+These tests are designed to test the CLI interface for each supported Librarian command
+on a local system. It tests the interface with docker using a fake repo and test docker
+image. These tests are run as presubmits and postsubmits via GitHub actions.
+
+Setup:
+
+```bash
+DOCKER_BUILDKIT=1 docker build \
+  -f ./testdata/e2e-test.Dockerfile \
+  -t test-image:latest \
+  .
+```
+
+Usage:
+
+```bash
+go test -tags e2e
+```
+
+## Integration Tests
+
+These tests are designed to test interactions with remote systems (e.g. GitHub). These
+tests are **NOT** run automatically as they create pull requests and branches.
+
+Usage:
+
+```bash
+TEST_GITHUB_TOKEN=<a personal access token> \
+  TEST_GITHUB_REPO=<URL of GitHub repo> \
+  go test ./...
+```
+
+Note: `TEST_GITHUB_TOKEN` must have write access to `TEST_GITHUB_REPO`.
+
+Note: These tests are skipped unless these environment variables are set.