Test examples of use (#9032)

# Description
Add `test_examples()` to `use` that verifies its examples. For that, add
the necessary definitions and the `PWD` to the `nu-cmd-lang` test
support.

Note: `let-env` is a `nu-command` and thus not accessible from the
`nu-cmd-lang` tests. So replace `let-env` with `$env.<var> =`. This
should be fine, since `let-env` is supposed to be removed before 1.0
anyway.


# User-Facing Changes
Nothing

# Question for reviewer
Is there any particular reason why so many core commands (parser
keywords) don't verify their examples with tests? E.g. `break`, `hide`,
`overlay use`, etc. (I think most of them?). If there is no particular
reason, I'd make a follow up PR to enable tests for as many of them as
possible.

crates/nu-cmd-lang/src/core_commands/use_.rs

@@ -20,6 +20,7 @@ impl Command for Use {
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("use")
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
+            .allow_variants_without_examples(true)
             .required("module", SyntaxShape::String, "Module or module file")
             .rest(
                 "members",
@@ -134,7 +135,7 @@ This command is a parser keyword. For details, check:
             },
             Example {
                 description: "Define a custom command that participates in the environment in a module and call it",
-                example: r#"module foo { export def-env bar [] { let-env FOO_BAR = "BAZ" } }; use foo bar; bar; $env.FOO_BAR"#,
+                example: r#"module foo { export def-env bar [] { $env.FOO_BAR = "BAZ" } }; use foo bar; bar; $env.FOO_BAR"#,
                 result: Some(Value::test_string("BAZ")),
             },
             Example {
@@ -160,3 +161,13 @@ This command is a parser keyword. For details, check:
         ]
     }
 }
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn test_examples() {
+        use super::Use;
+        use crate::test_examples;
+        test_examples(Use {})
+    }
+}