Add a hint in Thread Group UI about duration of test

@pmouawad ([Bug 62914](https://bz.apache.org/bugzilla//show_bug.cgi?id=62914&redirect=false)):
I often see on stackoverflow and during my training / consulting work a misunderstanding about duration of test when user:

- configures scheduler with a duration
- but keeps Loop Count to a limited number

The question is then, how come my test didn't last for the expected duration ?

To disambiguate this, I propose to add this note in UI:

If Loop Count is not -1 or Forever, duration will be min(Duration, Loop Count * iteration duration)

OS: All