Fix clippy warnings (#4176)

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

@@ -33,7 +33,7 @@ pub struct NuError {
     pub output: Option<Outcome>,
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Default)]
 pub struct NuProcess {
     pub arguments: Vec<OsString>,
     pub environment_vars: Vec<EnvironmentVariable>,
@@ -52,16 +52,6 @@ impl fmt::Display for NuProcess {
     }
 }
 
-impl Default for NuProcess {
-    fn default() -> Self {
-        Self {
-            arguments: vec![],
-            environment_vars: Vec::default(),
-            cwd: None,
-        }
-    }
-}
-
 impl NuProcess {
     pub fn arg<T: AsRef<OsStr>>(&mut self, arg: T) -> &mut Self {
         self.arguments.push(arg.as_ref().to_os_string());