[dyn_modules] add snapshot target for faster iteration.

This adds a target for running pkg/dynamic_modules/test/runner/main.dart from
an aot snapshot, useful when repeating runs for local iteration.

I'm not including it with other build targets or using it in the
test_matrix because each bot only runs this script once, so it doesn't
provide much savings there.

Change-Id: I20fe50a82d2a59aca6a0e697658b26b5b7da97c8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/480744
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>

utils/dynamic_module_runner/BUILD.gn

@@ -17,3 +17,8 @@ aot_snapshot("dynamic_module_runner_snapshot") {
       [ "--dynamic-interface=" + rebase_path(
             "$_dart_root/utils/dynamic_module_runner/dynamic_interface.yaml") ]
 }
+
+aot_snapshot("dynamic_modules_test_suite_snapshot") {
+  main_dart = "../../pkg/dynamic_modules/test/runner/main.dart"
+  name = "dynamic_modules_test_suite"
+}