Add test for const class references

modules/gdscript/tests/scripts/runtime/features/const_class_reference.gd

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+# https://github.com/godotengine/godot/issues/61636
+
+const External := preload("const_class_reference_external.notest.gd")
+
+class Class1:
+    class Class2:
+        pass
+
+const Class1Alias = Class1
+const Class1Class2Alias = Class1.Class2
+
+const ExternalAlias = External
+const ExternalClassAlias = External.Class
+
+func test():
+    pass

modules/gdscript/tests/scripts/runtime/features/const_class_reference.out

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+GDTEST_OK

modules/gdscript/tests/scripts/runtime/features/const_class_reference_external.notest.gd

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+class Class:
+    pass