test: wait for message from parent in embedding cctest

I’ve seen this fail a few times in CI, presumably because the
inspector commmand did not reach the child thread in time.
Explicitly waiting seems to solve that.

Refs: #30467

Backport-PR-URL: #35241
PR-URL: #32563
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

test/cctest/test_environment.cc

@@ -406,6 +406,7 @@ TEST_F(EnvironmentTest, InspectorMultipleEmbeddedEnvironments) {
 
       v8::Local<v8::Value> extracted_value = LoadEnvironment(
           environment,
+          "while (!global.variableFromParent) {}\n"
           "return global.variableFromParent;",
           std::move(data->inspector_parent_handle)).ToLocalChecked();