test: add tests for pre generation hook

brick/hooks/pubspec.yaml

@@ -7,4 +7,6 @@ dependencies:
   mason: ^0.1.0-dev.50
 
 dev_dependencies:
+  mocktail: ^1.0.0
+  test: ^1.19.2
   very_good_analysis: ^5.1.0

brick/hooks/test/pre_gen_test.dart

@@ -0,0 +1,50 @@
+import 'package:mason/mason.dart';
+import 'package:mocktail/mocktail.dart';
+import 'package:test/test.dart';
+
+import '../pre_gen.dart' as pre_gen;
+
+class _MockHookContext extends Mock implements HookContext {}
+
+void main() {
+  group('pre_gen', () {
+    late HookContext context;
+
+    setUp(() {
+      context = _MockHookContext();
+    });
+
+    group('application_id', () {
+      test('when specified is unmodified', () {
+        final vars = <String, String>{
+          'project_name': 'project_name',
+          'org_name': 'org_name',
+          'application_id': 'com.example.app',
+        };
+        when(() => context.vars).thenReturn(vars);
+
+        pre_gen.run(context);
+
+        expect(context.vars['application_id'], 'com.example.app');
+      });
+
+      test(
+        '''when not specified is set to `org_name + "." + project_name(snake-case)`''',
+        () {
+          final vars = <String, String>{
+            'project_name': 'Project Name',
+            'org_name': 'org_name',
+          };
+          when(() => context.vars).thenReturn(vars);
+
+          pre_gen.run(context);
+
+          expect(
+            context.vars['application_id'],
+            'org_name.project_name',
+          );
+        },
+      );
+    });
+  });
+}