GDScript: Add tests for highlighter #24138

Job	Run time
📊 Static checks / Code style, file formatting, and docs	1m 19s
🤖 Android / Template (target=template_release)	4m 46s
🏁 Windows / Editor (target=editor, tests=yes)	15m 42s
🐧 Linux / Editor w/ Mono (target=editor)	3m 40s
🏁 Windows / Template (target=template_release)	9m 27s
🍏 iOS / Template (target=template_release)	2m 29s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	24m 42s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	4m 9s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	8m 58s
🌐 Web / Template w/o threads (target=template_release, threads=no)	4m 7s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	9m 26s
🍎 macOS / Editor (target=editor, tests=yes)	5m 37s
🐧 Linux / Template w/ Mono (target=template_release)	3m 42s
🍎 macOS / Template (target=template_release)	2m 55s
🐧 Linux / Minimal template (target=template_release, everything disabled)	4m 25s
🪲 Godot CPP / Build and test Godot CPP	7m 0s
	1h 52m 24s

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