Add test case for F401 in __init__ files (#10364)

In preparation for https://github.com/astral-sh/ruff/pull/5845

crates/ruff_linter/src/rules/pyflakes/mod.rs

@@ -212,7 +212,11 @@ mod tests {
     fn init() -> Result<()> {
         let diagnostics = test_path(
             Path::new("pyflakes/__init__.py"),
-            &LinterSettings::for_rules(vec![Rule::UndefinedName, Rule::UndefinedExport]),
+            &LinterSettings::for_rules(vec![
+                Rule::UndefinedName,
+                Rule::UndefinedExport,
+                Rule::UnusedImport,
+            ]),
         )?;
         assert_messages!(diagnostics);
         Ok(())

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__init.snap

@@ -1,4 +1,17 @@
 ---
 source: crates/ruff_linter/src/rules/pyflakes/mod.rs
 ---
+__init__.py:1:8: F401 [*] `os` imported but unused
+  |
+1 | import os
+  |        ^^ F401
+2 | 
+3 | print(__path__)
+  |
+  = help: Remove unused import: `os`
 
+ℹ Safe fix
+1   |-import os
+2 1 | 
+3 2 | print(__path__)
+4 3 |