Use nu-cmd-lang default context for plugin tests (#12434)

# Description
@ayax79 added `nu-cmd-lang` as a dep for `nu-plugin-test-support` in
order to get access to `let`. Since we have the dep anyway now, we might
as well just add all of the lang commands - there aren't very many of
them and it would be less confusing than only `let` working.

# User-Facing Changes
- Can use some more core nu language features in plugin tests, like
loops and `do`

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting
- [ ] Might need to change something about the plugin testing section of
the book, since I think it says something about there only being the
plugin command itself available

crates/nu-plugin-test-support/tests/hello/mod.rs

@@ -73,3 +73,16 @@ fn test_an_example_with_the_wrong_result() -> Result<(), ShellError> {
     assert!(result.is_err());
     Ok(())
 }
+
+#[test]
+fn test_requiring_nu_cmd_lang_commands() -> Result<(), ShellError> {
+    use nu_protocol::Span;
+
+    let result = PluginTest::new("hello", HelloPlugin.into())?
+        .eval("do { let greeting = hello; $greeting }")?
+        .into_value(Span::test_data());
+
+    assert_eq!(Value::test_string("Hello, World!"), result);
+
+    Ok(())
+}