[AltTester Editor window] Add a tooltip on hovering the ALTTESTER define symbol setting

## Description

It would be helpful to have a tooltip on hovering the `Keep ALTTESTER symbol defined` setting (in _Build Settings_) to give more information on what it is actually useful for and to avoid confusion as the users are likely to find the information in the AltTester Editor rather than in the documentation.

## Additional info
- linked issue https://github.com/alttester/AltTester-Unity-SDK/issues/1078