Address lints from clippy for beta/nightly (#5709)

* Fix clippy lints in tests

* Replace `format!` in `.push_str()` with `write!`

Stylistically that might be a bit rough but elides an allocation.

Fallibility of allocation is more explicit, but ignored with `let _ =`
like in the clippy example:

https://rust-lang.github.io/rust-clippy/master/index.html#format_push_string

* Remove unused lifetime

* Fix macro crate relative import

* Derive `Eq` for `PartialEq` with `Eq` members

https://rust-lang.github.io/rust-clippy/master/index.html#derive_partial_eq_without_eq

* Remove unnnecessary `.to_string()` for Cow<str>

* Remove `.to_string()` for `tendril::Tendril`

Implements `Deref<Target = str>`

crates/nu-color-config/src/nu_style.rs

@@ -1,7 +1,7 @@
 use nu_ansi_term::{Color, Style};
 use serde::Deserialize;
 
-#[derive(Deserialize, PartialEq, Debug)]
+#[derive(Deserialize, PartialEq, Eq, Debug)]
 pub struct NuStyle {
     pub fg: Option<String>,
     pub bg: Option<String>,