add tests to polars unique (#12683)

# Description

I would like to help with `polars` plugin development and add tests to
all the `polars` command's existing params.

Since I have never written any lines of Rust, even though the task of
creating tests is relatively simple, I would like to ask for feedback to
ensure I did everything correctly here.

crates/nu_plugin_polars/src/dataframe/series/unique.rs

@@ -66,6 +66,59 @@ impl PluginCommand for Unique {
  .into_value(Span::test_data()),
  ),
  },
+ Example {
+ description: "Returns unique values in a subset of lazyframe columns",
+ example: "[[a b c]; [1 2 1] [2 2 2] [3 2 1]] | polars into-lazy | polars unique --subset [b c] | polars collect",
+ result: Some(
+ NuDataFrame::try_from_columns(
+ vec![
+ Column::new(
+ "a".to_string(),
+ vec![Value::test_int(1), Value::test_int(2)]
+ ),
+ Column::new(
+ "b".to_string(),
+ vec![Value::test_int(2), Value::test_int(2)]
+ ),
+ Column::new(
+ "c".to_string(),
+ vec![Value::test_int(1), Value::test_int(2)]
+ )
+ ],
+ None,
+ )
+ .expect("simple df for test should not fail")
+ .into_value(Span::test_data()),
+ ),
+ },
+ Example {
+ description: "Returns unique values in a subset of lazyframe columns",
+ example: r#"[[a b c]; [1 2 1] [2 2 2] [3 2 1]]
+ | polars into-lazy
+ | polars unique --subset [b c] --last
+ | polars collect"#,
+ result: Some(
+ NuDataFrame::try_from_columns(
+ vec![
+ Column::new(
+ "a".to_string(),
+ vec![Value::test_int(2), Value::test_int(3)]
+ ),
+ Column::new(
+ "b".to_string(),
+ vec![Value::test_int(2), Value::test_int(2)]
+ ),
+ Column::new(
+ "c".to_string(),
+ vec![Value::test_int(2), Value::test_int(1)]
+ )
+ ],
+ None,
+ )
+ .expect("simple df for test should not fail")
+ .into_value(Span::test_data()),
+ ),
+ },
  Example {
  description: "Creates a is unique expression from a column",
  example: "col a | unique",