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/path/expand_path.rs

@@ -1,8 +1,7 @@
-use std::path::PathBuf;
-
-use nu_test_support::playground::Playground;
-
 use nu_path::expand_path_with;
+use nu_test_support::playground::Playground;
+use pretty_assertions::assert_eq;
+use std::path::PathBuf;
 
 #[cfg(not(windows))]
 #[test]