[analysis_server] Fix flaky test

This test was writing a package_config after starting the analysis server which causes the context roots to be rebuilt (during which there is a period when there are no roots/drivers), and then not waiting.

This change creates the file before server initialisation.

Change-Id: Id0111f16200a905dd538e217199133122be84246
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/319801
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/test/lsp_over_legacy/document_color_test.dart

@@ -18,8 +18,8 @@ void main() {
 @reflectiveTest
 class DocumentColorTest extends LspOverLegacyTest {
   @override
-  Future<void> setUp() async {
-    await super.setUp();
+  void createDefaultFiles() {
+    super.createDefaultFiles();
     writeTestPackageConfig(flutter: true);
   }