feat(core): allow the MockRuntime to call a command and get a return value + fix doctest

[vdang-crabnebula]

Similar to #8228 but for dev branch.
This PR improves on the current MockRuntime by giving it a function to call a Tauri command and get back the command result.
The end goal is to use these changes to facilitate fuzzing Tauri applications.

This PR changes:

• Add 2 functions to the MockRuntime
• Fix the doctests of the tauri::test module
• No breaking changes

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information