Add a regression test

This should help keep at least the "starting with failed modules" source
of the "module defined in multiple files" bug gone.

ghcid-ng/tests/failed_modules.rs

@@ -0,0 +1,35 @@
+use test_harness::fs;
+use test_harness::test;
+use test_harness::GhcidNgBuilder;
+
+/// Test that `ghcid-ng` can start with compile errors.
+///
+/// This is a regression test for [#43](https://github.com/MercuryTechnologies/ghcid-ng/issues/43).
+#[test]
+async fn can_start_with_failed_modules() {
+    let module_path = "src/MyModule.hs";
+    let mut session = GhcidNgBuilder::new("tests/data/simple")
+        .before_start(move |path| async move {
+            fs::replace(path.join(module_path), "example :: String", "example :: ()").await
+        })
+        .start()
+        .await
+        .expect("ghcid-ng starts");
+    let module_path = session.path(module_path);
+
+    session
+        .get_log("Compilation failed")
+        .await
+        .expect("ghcid-ng fails to load with errors");
+
+    session.wait_until_ready().await.expect("ghcid-ng loads");
+
+    fs::replace(&module_path, "example :: ()", "example :: String")
+        .await
+        .unwrap();
+
+    session
+        .get_log("Compilation succeeded")
+        .await
+        .expect("ghcid-ng reloads fixed modules");
+}