add e2e testing (#167)

Reviewed-on: #167

integration_test/helpers/pump_until.dart

@@ -0,0 +1,56 @@
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:flutter_test/flutter_test.dart";
+import "package:material_ui/material_ui.dart";
+import "package:nexus/widgets/common/error_dialog.dart";
+
+// Custom because `patrol_finder` gives me jQuery vibes, and that's Not GoodTM
+extension PumpUntil on WidgetTester {
+  /// Pumps until [finder] finds something, failing if it doesn't within 30
+  /// seconds, or if the app shows an error.
+  Future<void> pumpUntil(Finder finder) async {
+    const timeout = Duration(seconds: 30);
+
+    String textsIn(Finder finder) => widgetList<Text>(
+      find.descendant(of: finder, matching: find.byType(Text)),
+    ).map((text) => text.data).nonNulls.join("\n");
+
+    final stopwatch = Stopwatch()..start();
+    while (stopwatch.elapsed < timeout) {
+      await pump(const .new(milliseconds: 100));
+
+      if (finder.evaluate().isNotEmpty) return;
+
+      final errorDialog = find.byType(ErrorDialog);
+      if (errorDialog.evaluate().isNotEmpty) {
+        fail(
+          "The app showed an error while waiting for $finder:\n"
+          "${textsIn(errorDialog)}",
+        );
+      }
+    }
+
+    final snackBar = find.byType(SnackBar);
+    final shown = find
+        .byType(RichText)
+        .hitTestable()
+        .evaluate()
+        .map((element) => (element.widget as RichText).text.toPlainText())
+        .where((text) => text.trim().isNotEmpty)
+        .toIList()
+        .removeDuplicates()
+        .join(" | ");
+    fail(
+      "Timed out after $timeout waiting for $finder"
+      "${snackBar.evaluate().isEmpty ? "" : "\nSnack bar: ${textsIn(snackBar)}"}"
+      "\nShown: $shown",
+    );
+  }
+
+  /// Taps [finder] once it can be tapped, like after animating in.
+  Future<void> tapWhenShown(Finder finder) async {
+    final tappable = finder.hitTestable();
+    await pumpUntil(tappable);
+    await tap(tappable.first);
+    await pump();
+  }
+}