Implement TestSandbox.writeTextFile

[bajtos]

This is a follow-up for the spike #3617.

Implement a helper for writing text files to the test sandbox.

Example implementation:

https://github.com/strongloop/loopback-next/blob/2848a1b3e4c62da88cfd3edc966e8267768cf326/packages/testlab/src/test-sandbox.ts#L124-L129

Acceptance criteria

• Implementation
• API documentation
• Automated tests

[hacksparrow]

Closed via #3824