website: add guide for testing

As required by <https://www.bestpractices.dev/en/projects/6505>:

> Quality
> [...]
> New functionality testing
>
> - The project MUST have a general policy (formal or not) that as major
>   new functionality is added to the software produced by the project,
>   tests of that functionality should be added to an automated test
>   suite.
>
> - The project MUST have evidence that the test_policy for adding tests
>   has been adhered to in the most recent major changes to the software
>   produced by the project.
>
> - It is SUGGESTED that this policy on adding tests (see test_policy) be
>   documented in the instructions for change proposals.

Signed-off-by: Akihiro Suda <[email protected]>

Author: AkihiroSuda

website/content/en/docs/Community/Contributing/_index.md

@@ -21,6 +21,10 @@ See also <https://github.com/cncf/foundation/blob/main/allowed-third-party-licen
 
 Pull requests can be submitted to <https://github.com/lima-vm/lima/pulls>.
 
+It is highly suggested to add [tests](../../dev/testing/) for every non-trivial pull requests.
+A test can be implemented as a unit test rather than an integration test when it is possible,
+to avoid slowing the integration test CI.
+
 ## Merging pull requests
 
 [Committers](../governance) can merge pull requests.

website/content/en/docs/dev/Testing/_index.md

@@ -0,0 +1,41 @@
+---
+title: Testing
+weight: 20
+---
+
+## Unit tests
+
+The unit tests are written in Go and can be executed with the following commands:
+
+```bash
+go test -v ./...
+```
+
+The unit tests do not execute actual virtual machines.
+
+## Integration tests
+
+The integration tests incurs actual execution of virtual machines.
+
+The integration tests are written in bash and partially in Perl.
+
+Use [`hack/test-templates.sh`](https://github.com/lima-vm/lima/blob/master/hack/test-templates.sh)
+to execute integration tests, with a virtual machine template file, e.g.,:
+
+```bash
+./hack/test-templates.sh ./templates/default.yaml
+./hack/test-templates.sh ./templates/fedora.yaml
+./hack/test-templates.sh ./hack/test-templates/test-misc.yaml
+```
+
+## CI
+
+[`.github/workflows/test.yml`](https://github.com/lima-vm/lima/blob/master/.github/workflows/test.yml)
+executes the unit tests and the integration tests on the GitHub Actions with the ["Tier 1"](../../templates/) templates.
+
+Most integration tests are executed on Linux runners, as macOS runners are slow and flaky.
+
+The tests about macOS-specific features (e.g., vz and vmnet) are still executed on macOS runners.
+
+Currently, the Intel version of macOS is used, as the ARM version of macOS on GitHub Actions still
+do not support nested virtualization.