Doc: Clarify TestContext lifecycle and hook execution order

[Rawal27]

Summary

Improve the TestContext documentation by adding:

• A TestContext lifecycle section describing when a TestContext instance is created and how it relates to subtests.
• A small example showing that parent tests and subtests each receive their own TestContext instance.
• An Execution order section documenting the order in which test hooks are executed.

These additions make the TestContext documentation easier to understand for users who are new to the test runner, while complementing the existing API reference without changing any behavior.

Documentation changes

• Add TestContext lifecycle
• Add Example
• Add Execution order

No functional changes.

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner