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-test-support/src/macros.rs

@@ -121,7 +121,7 @@ macro_rules! nu_with_plugins {
         pub use std::error::Error;
         pub use std::io::prelude::*;
         pub use std::process::{Command, Stdio};
-        pub use crate::NATIVE_PATH_ENV_VAR;
+        pub use $crate::NATIVE_PATH_ENV_VAR;
 
         let commands = &*format!(
             "

crates/nu-test-support/src/playground/director.rs

@@ -2,6 +2,7 @@ use super::nu_process::*;
 use super::EnvironmentVariable;
 use std::ffi::OsString;
 use std::fmt;
+use std::fmt::Write;
 
 #[derive(Default, Debug)]
 pub struct Director {
@@ -94,7 +95,7 @@ impl Executable for Director {
                         if !commands.is_empty() {
                             commands.push_str("| ");
                         }
-                        commands.push_str(&format!("{}\n", pipeline));
+                        let _ = writeln!(commands, "{}", pipeline);
                     }
                 }