Use pretty_assertions in the root crate (#8818)

# Description

This PR is just a minor development improvement. While working on
another feature, I noticed that the root crate lists the super useful
`pretty_assertions` in the root crate but doesn't use it in most tests.
With this change `pretty_assertions::assert_eq!` is used instead of
`core::assert_eq!` for better diffs when debugging the tests.

I thought of adding the dependency to other crates but I decided not to
since I didn't want a huge disruptive PR :)

tests/plugins/custom_values.rs

@@ -1,4 +1,5 @@
 use nu_test_support::nu_with_plugins;
+use pretty_assertions::assert_eq;
 
 #[test]
 fn can_get_custom_value_from_plugin_and_instantly_collapse_it() {