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/shell/pipeline/commands/external.rs

@@ -1,4 +1,5 @@
 use nu_test_support::nu;
+use pretty_assertions::assert_eq;
 
 #[cfg(feature = "which-support")]
 #[test]

tests/shell/pipeline/commands/internal.rs

@@ -2,6 +2,7 @@ use nu_test_support::fs::Stub::FileWithContentToBeTrimmed;
 use nu_test_support::nu;
 use nu_test_support::pipeline;
 use nu_test_support::playground::Playground;
+use pretty_assertions::assert_eq;
 
 #[test]
 fn takes_rows_of_nu_value_strings_and_pipes_it_to_stdin_of_external() {

tests/shell/pipeline/mod.rs

@@ -1,6 +1,7 @@
 mod commands;
 
 use nu_test_support::nu;
+use pretty_assertions::assert_eq;
 
 #[test]
 fn doesnt_break_on_utf8() {