GDScript: Add tests for highlighter

[dalexeev]

• Closes No unit tests for GDScript code highlighter #88887.

[Mickeon]

Yyyyyeah, we need to discuss this somewhere. The arbitrary character count for each identifier is very prone to confusion and hard to decipher. At the same time, so would be using a single character.

[Mickeon]

I think this system would greatly benefit from the ability to define highlighter placeholders for each individual test, but I have absolutely no idea on how to go about that.
No matter what suggestions one could have about the current choices, having to maintain COLOR_INFO could be troublesome, and it's somewhat tedious to decipher when reading and writing these tests.

[Mickeon]

There's an alternative approach I'd like to propose. Instead of printing the highlighting per-line, it could be printed for each type of highlighting. As a silly mockup based on the PR's test:

N at:
	1: 5, 
	2: 5, 10, [...]
symbol_color at:
	1: 4, 7, 11, 15, [...]
keyword_color at:
	1: 8, 
	2: 1, [...]

[Wierdox]

Assuming my PR is added, here's some ideas for tests of StringName/&&/&. I don't have experience in this area though.

# & should be symbol colored.
true \
&& true
1 \
& 4
& "error" # If your test allows errors.

# & next to quote should be StringName colored, unless it is exactly '&&""'.
var sname := &''+&""

sname&&&""
sname&&""