Clean the application state when loading a snippet from SnippetList

[solkaz]

Description

Refactors <CodesplainAppBar /> to dispatch actions to clean the application state when a snippet is selected from the <SnippetMenu /> Also adds test utilities to mock the Redux store and React-Router.

Motivation and Context

Fixes #505

Checklist:

• [x] My code follows the code style of this project.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

Browsers

• [x] Chrome
• [x] Firefox
• [x] Safari
• [ ] Mobile:
• [ ] Other: