Fix Rust test for logical plan optimizer for categoricals

[stinodego]

There's a Rust test for the logical plan optimizer for categorical columns. It should do the following:

• Make sure categorical columns are not cast to Utf8 when doing an equality check with a string literal.
• Make sure categorical columns ARE cast to Utf8 when doing an addition with a string literal

See code here:

polars/polars/polars-lazy/polars-plan/src/logical_plan/optimizer/type_coercion/mod.rs

Lines 518 to 541 in a1d5a22

	#[cfg(test)]
	#[cfg(feature = "dtype-categorical")]
	mod test {
	use polars_core::prelude::*;
	use super::*;
	use crate::prelude::*;
	#[test]
	fn test_categorical_utf8() {
	let mut rules: Vec<Box<dyn OptimizationRule>> = vec![Box::new(TypeCoercionRule {})];
	let schema = Schema::from_iter([Field::new("fruits", DataType::Categorical(None))]);
	let expr = col("fruits").eq(lit("somestr"));
	let out = optimize_expr(expr.clone(), schema.clone(), &mut rules);
	// we test that the fruits column is not casted to utf8 for the comparison
	assert_eq!(out, expr);
	let expr = col("fruits") + (lit("somestr"));
	let out = optimize_expr(expr, schema, &mut rules);
	let expected = col("fruits").cast(DataType::Utf8) + lit("somestr");
	assert_eq!(out, expected);
	}
	}

However, the test does not compile. It is outdated and hasn't been run in a while. Still, we want to test this functionality. So the test should be rewritten for the latest syntax.

[ritchie46]

Thanks! :)