Change flutter_tools tests to run via pub (#8698)

`all.dart` is no longer needed. Furthermore, it causes tests to
be skipped, or to silently fail to run anything.

Fixes #7941
diff --git a/packages/flutter_tools/test/packages_test.dart b/packages/flutter_tools/test/packages_test.dart
index d204d82..a3484a3 100644
--- a/packages/flutter_tools/test/packages_test.dart
+++ b/packages/flutter_tools/test/packages_test.dart
@@ -6,6 +6,7 @@
 
 import 'package:args/command_runner.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
+import 'package:flutter_tools/src/cache.dart';
 import 'package:flutter_tools/src/commands/create.dart';
 import 'package:flutter_tools/src/commands/packages.dart';
 import 'package:test/test.dart';
@@ -17,6 +18,10 @@
   group('packages', () {
     Directory temp;
 
+    setUpAll(() {
+      Cache.disableLocking();
+    });
+
     setUp(() {
       temp = fs.systemTempDirectory.createTempSync('flutter_tools');
     });