No unit tests for GDScript code highlighter

[nlupugla]

Tested versions

4.3.dev and all previous

System information

N/A

Issue description

The GDScript unit tests currently do not cover syntax highlighting, despite covering other editor-related features like code completion. There have recently been some fixes to syntax highlighting: #86176 and the changes have already introduced one known regression: #88876. In light of this, a small suite of unit tests to cover syntax highlighting seems appropriate. It would also probably be a good first issue for a new contributor I'm guessing.

Steps to reproduce

N/A

Minimal reproduction project (MRP)

N/A