Write basic guide on unit testing with moor

docs/content/en/docs/testing.md

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+---
+title: "Testing"
+description: Guide on writing unit tests for moor databases
+---
+
+Flutter apps using moor can always be tested with [integration tests](https://flutter.dev/docs/cookbook/testing/integration/introduction)
+running on a real device. This guide focusses on writing unit tests for a database written in moor.
+Those tests can be run and debugged on your computer without additional setup, you don't a
+physical device to run them.
+
+## Setup
+In addition to the moor dependencies you already have, add `moor_ffi` as a dev dependency. If you're already using `moor_ffi`
+as a regular dependency, you can skip this.
+```yaml
+dev_dependencies:
+  moor_ffi: ^0.1.0 # or latest version
+```
+
+For this guide, we're going to test a very simple database that stores user names. The only important change from a regular moor
+database is the constructor: We make the `QueryExecutor` argument explicit instead of having a no-args constructor that passes
+a `FlutterQueryExector` to the superclass.
+```dart
+import 'package:moor/moor.dart';
+
+part 'database.g.dart';
+
+class Users extends Table {
+  IntColumn get id => integer().autoIncrement()();
+  TextColumn get name => text()();
+}
+
+@UseMoor(tables: [Users])
+class MyDatabase extends _$MyDatabase {
+  MyDatabase(QueryExecutor e) : super(e);
+
+  @override
+  int get schemaVersion => 1;
+
+  /// Creates a user and returns their id
+  Future<int> createUser(String name) {
+    return into(users).insert(UsersCompanion.insert(name: name));
+  }
+
+  /// Changes the name of a user with the [id] to the [newName].
+  Future<void> updateName(int id, String newName) {
+    return update(users).replace(User(id: id, name: newName));
+  }
+
+  Stream<User> watchUserWithId(int id) {
+    return (select(users)..where((u) => u.id.equals(id))).watchSingle();
+  }
+}
+```
+
+## Writing tests
+
+We can create an in-memory version of the database by using a 
+`VmDatabase.memory()` instead of a `FlutterQueryExecutor`. A good
+place to open the database is the `setUp` and `tearDown` methods from
+`package:test`:
+```dart
+import 'package:moor_ffi/moor_ffi.dart';
+import 'package:test/test.dart';
+// the file defined above, you can test any moor database of course
+import 'database.dart'; 
+
+void main() {
+  MyDatabase database;
+
+  setUp(() {
+    database = MyDatabase(VmDatabase.memory());
+  });
+  tearDown(() async {
+    await database.close();
+  });
+}
+```
+
+With that setup in place, we can finally write some tests:
+```dart
+test('users can be created', () async {
+  final id = await database.createUser('some user');
+  final user = await database.watchUserWithId(id).first;
+
+  expect(user.name, 'some user');
+});
+
+test('stream emits a new user when the name updates', () async {
+  final id = await database.createUser('first name');
+
+  final expectation = expectLater(
+    database.watchUserWithId(id).map((user) => user.name),
+    emitsInOrder(['first name', 'changed name']),
+  );
+
+  await database.updateName(id, 'changed name');
+  await expectation;
+});
+```