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 :)

src/tests/test_config_path.rs

@@ -1,4 +1,5 @@
 use nu_test_support::nu;
+use pretty_assertions::assert_eq;
 use std::fs;
 use std::path::Path;
 

src/tests/test_custom_commands.rs

@@ -1,5 +1,6 @@
 use crate::tests::{fail_test, run_test, run_test_contains, TestResult};
 use nu_test_support::nu;
+use pretty_assertions::assert_eq;
 
 #[test]
 fn no_scope_leak1() -> TestResult {