Rename GDScript test script filenames to use snake_case

modules/gdscript/tests/scripts/parser/features/semicolon_as_end_statement.gd

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+func test():
+    print("A"); print("B")

modules/gdscript/tests/scripts/parser/features/semicolon_as_end_statement.out

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+GDTEST_OK
+A
+B

modules/gdscript/tests/scripts/parser/features/trailing_comma_in_function_args.gd

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+# See https://github.com/godotengine/godot/issues/41066.
+
+func f(p, ): ## <-- no errors
+	print(p)
+
+func test():
+	f(0, ) ## <-- no error

modules/gdscript/tests/scripts/parser/features/trailing_comma_in_function_args.out

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+GDTEST_OK
+0

modules/gdscript/tests/scripts/parser/features/variable_declaration.gd

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+var a # No init.
+var b = 42 # Init.
+
+func test():
+	var c # No init, local.
+	var d = 23 # Init, local.
+
+	a = 1
+	c = 2
+
+	prints(a, b, c, d)
+	print("OK")

modules/gdscript/tests/scripts/parser/features/variable_declaration.out

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+GDTEST_OK
+>> WARNING
+>> Line: 5
+>> UNASSIGNED_VARIABLE
+>> The variable 'c' was used but never assigned a value.
+1 42 2 23
+OK