tests: improve just watch

[laniakea64]

Proposed fix for #65 . Also update README.md to reflect these changes, mention the ftdetect tests, and clarify the nature of the parameter passed to just run.

[laniakea64]

Hi @NoahTheDuke so the reasons I proposed this PR for review instead of going ahead pushing to main are:

1. I don't know the history of just watch, as in why it was originally done the way it was? So not sure how this change meshes with that, or if that's important.

2. I don't know if the change in behavior of just watch would break your workflow 😮

3. Not sure if the changes to README.md make it too verbose?

Would appreciate your take on these when you get to review this, thanks.

[NoahTheDuke]

just watch was most useful when I was iterating on the syntax. I'd make a change and the tests would rerun, so i could see the output immediately. I don't know how useful it is now that the syntax is fairly stable.

I don't have any opinions on the code at this point, it's been a while since I had to touch any of this, so I welcome your changes/opinions as the more active dev.

[laniakea64]

Thanks @NoahTheDuke , in that case I'll go ahead and merge this.

I don't know how useful it is now that the syntax is fairly stable.

Although I haven't used just watch much, some things in the syntax are tricky to get right, and an updated just watch could be nice to have for those cases.