Remove duplicate tests from test_const_evaluator_scalar_functions

[HaoYang670]

Describe the bug
I find that we have 2 same tests:

        // check that non foldable arguments are folded
        // to_timestamp(a) --> to_timestamp(a) [no rewrite possible]
        let expr = Expr::ScalarFunction {
            args: vec![col("a")],
            fun: BuiltinScalarFunction::ToTimestamp,
        };
        test_evaluate(expr.clone(), expr);


        // check that non foldable arguments are folded
        // to_timestamp(a) --> to_timestamp(a) [no rewrite possible]
        let expr = Expr::ScalarFunction {
            args: vec![col("a")],
            fun: BuiltinScalarFunction::ToTimestamp,
        };
        test_evaluate(expr.clone(), expr);

https://github.com/apache/arrow-datafusion/blob/master/datafusion/src/optimizer/simplify_expressions.rs#L1026-L1040

To Reproduce

Expected behavior
Remove one of the test or add some comment about why we need 2 same tests.

Additional context

[HaoYang670]

Another that I find
https://github.com/apache/arrow-datafusion/blob/master/datafusion/src/optimizer/simplify_expressions.rs#L1042-L1054

        // volatile / stable functions should not be evaluated
        // rand() + (1 + 2) --> rand() + 3
        let fun = BuiltinScalarFunction::Random;
        assert_eq!(fun.volatility(), Volatility::Volatile);
        let rand = Expr::ScalarFunction { args: vec![], fun };
        let expr = rand.clone() + (lit(1) + lit(2));
        let expected = rand + lit(3);
        test_evaluate(expr, expected);


        // parenthesization matters: can't rewrite
        // (rand() + 1) + 2 --> (rand() + 1) + 2)
        let fun = BuiltinScalarFunction::Random;
        assert_eq!(fun.volatility(), Volatility::Volatile);

We test BuiltinScalarFunction::Random.volatility() twice.

[bridyash13]

Hey @HaoYang670 can I work on this issue?

[HaoYang670]

Sure. Thank you, @bridyash13

[jackwener]

I found the problem to be a very simple one, and it didn't seem to be progressing. so I fix it.

BTW, I search the bug label and find it, and it shouldn't be a bug but an enhancement.

[jackwener]

I make a stupid mistake....ignored the supplement following.😅 I will add another PR......