add e2e testing (#167)

Reviewed-on: #167

integration_test/helpers/threads.dart

@@ -0,0 +1,34 @@
+import "package:flutter_test/flutter_test.dart";
+import "package:material_ui/material_ui.dart";
+
+import "pump_until.dart";
+
+extension MessageFinders on CommonFinders {
+  Finder message(String body) => textContaining(body, findRichText: true);
+}
+
+extension RelativeFinders on Finder {
+  Finder inside(Type type) =>
+      find.descendant(of: find.byType(type), matching: this);
+
+  /// The [InkWell] ancestor around what this finds
+  Finder get inkWell => find.ancestor(of: this, matching: find.byType(InkWell));
+}
+
+extension ReplyCounts on WidgetTester {
+  /// Waits for a reply count in widgets of [type], then checks it, so a wrong
+  /// count fails straight away with what was shown.
+  Future<void> expectReplyCount(Type type, int expected) async {
+    final repliesLabel = RegExp(r"^(\d+) repl(?:y|ies)$");
+    final labels = find.textContaining(repliesLabel).inside(type);
+    await pumpUntil(labels);
+
+    final shown = widgetList<Text>(labels)
+        .map((label) => label.data)
+        .nonNulls
+        .map(repliesLabel.firstMatch)
+        .nonNulls
+        .map((match) => int.parse(match[1]!));
+    expect(shown, everyElement(expected), reason: "Reply count in $type");
+  }
+}