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/second_custom_value.rs

@@ -1,7 +1,9 @@
+use std::cmp::Ordering;
+
 use nu_protocol::{CustomValue, ShellError, Span, Value};
 use serde::{Deserialize, Serialize};
 
-#[derive(Clone, Debug, Serialize, Deserialize)]
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord)]
 pub struct SecondCustomValue {
     pub(crate) something: String,
 }
@@ -59,6 +61,16 @@ impl CustomValue for SecondCustomValue {
         ))
     }
 
+    fn partial_cmp(&self, other: &Value) -> Option<Ordering> {
+        if let Value::CustomValue { val, .. } = other {
+            val.as_any()
+                .downcast_ref()
+                .and_then(|other: &SecondCustomValue| PartialOrd::partial_cmp(self, other))
+        } else {
+            None
+        }
+    }
+
     fn as_any(&self) -> &dyn std::any::Any {
         self
     }