Add Testing user guide article

[ntrogh]

Based on https://github.com/microsoft/vscode/issues/210152

Add a new article in User Guide that outlines the common testing features in VS Code. Currently, testing functionality is covered only in the language-specific sections of the docs, such as C#, Python, or Java.

Cover topics such as:

• Test Explorer view
• Run tests
• Debug tests
• Test coverage
• Test results
• ..