Reimplement TestTaskRunner on top of UnixTaskRunner

Change-Id: Icc8e61ac81b17bb421e6bc879b354ad58f79cda7
diff --git a/base/unix_task_runner_unittest.cc b/base/unix_task_runner_unittest.cc
index 477e991..14f358f 100644
--- a/base/unix_task_runner_unittest.cc
+++ b/base/unix_task_runner_unittest.cc
@@ -331,6 +331,17 @@
   thread.join();
 }
 
+TEST(UnixTaskRunner, IsIdleForTesting) {
+  UnixTaskRunner task_runner;
+  task_runner.PostTask(
+      [&task_runner] { EXPECT_FALSE(task_runner.IsIdleForTesting()); });
+  task_runner.PostTask([&task_runner] {
+    EXPECT_TRUE(task_runner.IsIdleForTesting());
+    task_runner.Quit();
+  });
+  task_runner.Run();
+}
+
 }  // namespace
 }  // namespace base
 }  // namespace perfetto