Isolate tests from user config (#12437)

# Description
This is an attempt to isolate the unit tests from whatever might be in
the user's config. If the
user's config is broken in some way or incompatible with this version
(for example, especially if
there are plugins that aren't built for this version), tests can
spuriously fail.

This makes tests more reliably pass the same way they would on CI even
if the user has config, and
should also make them run faster.

I think this is _good enough_, but I still think we should have a
specific config dir env variable for nushell specifically (rather than
having to use `XDG_CONFIG_HOME`, which would mess with other things) and
then we can just have `nu-test-support` set that to a temporary dir
containing the shipped default config files.

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

src/run.rs

@@ -146,57 +146,69 @@ pub(crate) fn run_file(
 ) -> Result<(), miette::ErrReport> {
     trace!("run_file");
     let mut stack = nu_protocol::engine::Stack::new();
-    let start_time = std::time::Instant::now();
 
-    #[cfg(feature = "plugin")]
-    read_plugin_file(
-        engine_state,
-        &mut stack,
-        parsed_nu_cli_args.plugin_file,
-        NUSHELL_FOLDER,
-    );
-    perf(
-        "read plugins",
-        start_time,
-        file!(),
-        line!(),
-        column!(),
-        use_color,
-    );
-
-    let start_time = std::time::Instant::now();
-    // only want to load config and env if relative argument is provided.
-    if parsed_nu_cli_args.env_file.is_some() {
-        config_files::read_config_file(engine_state, &mut stack, parsed_nu_cli_args.env_file, true);
-    } else {
-        config_files::read_default_env_file(engine_state, &mut stack)
-    }
-    perf(
-        "read env.nu",
-        start_time,
-        file!(),
-        line!(),
-        column!(),
-        use_color,
-    );
-
-    let start_time = std::time::Instant::now();
-    if parsed_nu_cli_args.config_file.is_some() {
-        config_files::read_config_file(
+    // if the --no-config-file(-n) option is NOT passed, load the plugin file,
+    // load the default env file or custom (depending on parsed_nu_cli_args.env_file),
+    // and maybe a custom config file (depending on parsed_nu_cli_args.config_file)
+    //
+    // if the --no-config-file(-n) flag is passed, do not load plugin, env, or config files
+    if parsed_nu_cli_args.no_config_file.is_none() {
+        let start_time = std::time::Instant::now();
+        #[cfg(feature = "plugin")]
+        read_plugin_file(
             engine_state,
             &mut stack,
-            parsed_nu_cli_args.config_file,
-            false,
+            parsed_nu_cli_args.plugin_file,
+            NUSHELL_FOLDER,
+        );
+        perf(
+            "read plugins",
+            start_time,
+            file!(),
+            line!(),
+            column!(),
+            use_color,
+        );
+
+        let start_time = std::time::Instant::now();
+        // only want to load config and env if relative argument is provided.
+        if parsed_nu_cli_args.env_file.is_some() {
+            config_files::read_config_file(
+                engine_state,
+                &mut stack,
+                parsed_nu_cli_args.env_file,
+                true,
+            );
+        } else {
+            config_files::read_default_env_file(engine_state, &mut stack)
+        }
+        perf(
+            "read env.nu",
+            start_time,
+            file!(),
+            line!(),
+            column!(),
+            use_color,
+        );
+
+        let start_time = std::time::Instant::now();
+        if parsed_nu_cli_args.config_file.is_some() {
+            config_files::read_config_file(
+                engine_state,
+                &mut stack,
+                parsed_nu_cli_args.config_file,
+                false,
+            );
+        }
+        perf(
+            "read config.nu",
+            start_time,
+            file!(),
+            line!(),
+            column!(),
+            use_color,
         );
     }
-    perf(
-        "read config.nu",
-        start_time,
-        file!(),
-        line!(),
-        column!(),
-        use_color,
-    );
 
     // Regenerate the $nu constant to contain the startup time and any other potential updates
     let nu_const = create_nu_constant(engine_state, input.span().unwrap_or_else(Span::unknown))?;