Automated tests for string generation input validation

[jbellik]

Desired behavior, which I think has been achieved:

When the user clicks "Generate trees":

• [x] no warnings if the user doesn't add anything to the "Generate combinations and permutations" section
• [x] display warning if the user adds something to the "Generate combinations and permutations" section but generates trees with it closed
• [x] validation that min < max < 10, and both min and max are present and are numbers; display errors if any of these conditions is not met

When the user clicks "Generate terminal strings" (which entails that the "Generate combinations and permutations" section is open):

• [x] display error if no inputs are present
• [x] validation that min < max < 10, and both min and max are present and are numbers; display errors if any of these conditions is not met

This behavior is fairly complicated so it would be good to automate some tests for it.

[jbellik]

Refer to https://github.com/syntax-prosody-ot/main/pull/474 for the other existing example of Mocha tests with interface1.html. You may also be able to come up with a better way of handling interface testing, though. Feel free to try something different.