[ty] Override home directory in ty tests (#23724)

Closes astral-sh/ty#2942.

Author: charliermarsh

crates/ty/tests/cli/main.rs

@@ -383,12 +383,8 @@ fn user_configuration() -> anyhow::Result<()> {
         ),
     ])?;
 
-    let config_directory = case.root().join("home/.config");
-    let config_env_var = if cfg!(windows) {
-        "APPDATA"
-    } else {
-        "XDG_CONFIG_HOME"
-    };
+    let config_directory = case.user_config_directory();
+    let config_env_var = user_config_directory_env_var();
 
     assert_cmd_snapshot!(
         case.command().current_dir(case.root().join("project")).env(config_env_var, config_directory.as_os_str()),
@@ -888,13 +884,16 @@ impl CliTest {
         // Canonicalize the tempdir path because macos uses symlinks for tempdirs
         // and that doesn't play well with our snapshot filtering.
         // Simplify with dunce because otherwise we get UNC paths on Windows.
-        let project_dir = dunce::simplified(
+        let temp_dir_path = dunce::simplified(
             &temp_dir
                 .path()
                 .canonicalize()
-                .context("Failed to canonicalize project path")?,
+                .context("Failed to canonicalize temporary directory path")?,
         )
         .to_path_buf();
+        let project_dir = temp_dir_path.join("project");
+        std::fs::create_dir_all(&project_dir)
+            .with_context(|| format!("Failed to create directory `{}`", project_dir.display()))?;
 
         let mut settings = insta::Settings::clone_current();
         settings.add_filter(&tempdir_filter(&project_dir), "<temp_dir>/");
@@ -1016,9 +1015,21 @@ impl CliTest {
 
         // Unset all environment variables because they can affect test behavior.
         command.env_clear();
+        // Point user config discovery at a test-local directory to avoid picking up host config.
+        command.env(
+            user_config_directory_env_var(),
+            self.user_config_directory(),
+        );
 
         command
     }
+
+    fn user_config_directory(&self) -> PathBuf {
+        self.project_dir
+            .parent()
+            .expect("project directory always has a parent")
+            .join("home/.config")
+    }
 }
 
 fn tempdir_filter(path: &Path) -> String {
@@ -1032,3 +1043,11 @@ fn site_packages_filter(python_version: &str) -> String {
         format!("lib/python{}/site-packages", regex::escape(python_version))
     }
 }
+
+fn user_config_directory_env_var() -> &'static str {
+    if cfg!(windows) {
+        "APPDATA"
+    } else {
+        "XDG_CONFIG_HOME"
+    }
+}