Add example tests (nu-plugin-test-support) for plugins in repo (#12281)

# Description

Uses the new `nu-plugin-test-support` crate to test the examples of
commands provided by plugins in the repo.

Also fixed some of the examples to pass.

# User-Facing Changes

- Examples that are more guaranteed to work

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

crates/nu_plugin_custom_values/src/generate2.rs

@@ -54,3 +54,11 @@ impl SimplePluginCommand for Generate2 {
         }
     }
 }
+
+#[test]
+fn test_examples() -> Result<(), nu_protocol::ShellError> {
+    use nu_plugin_test_support::PluginTest;
+
+    PluginTest::new("custom_values", crate::CustomValuePlugin.into())?
+        .test_command_examples(&Generate2)
+}