message: Add a note explaining in more detail the current and potential benefits of a common TestBase class.
diff: namespace Envoy {
  // Provides a common test-base class for all tests in Envoy to use. This offers
  -// a place to put hooks we'd like to run on every tests.
  +// a place to put hooks we'd like to run on every test. There's currently a
  +// check that all test-scoped singletons have been destroyed. A test-scoped
  +// singleton might remain at the end of a test if it's transitively referenced
  +// by a leaked structure or a static.
  +//
  +// In the future, we can also add:
  +// - a test-specific ThreadFactory that enables us to verify there are no
  +// outstanding threads at the end of each thread.
  +// - a check that no more bytes of memory are allocated at the end of a test
  +// than there were at the start of it. This is likely to fail in a few
  +// places when introduced, but we could add known test overrides for this.
  +//
  +// Note: nothing compute-intensive should be put in this test-class, as it will
  +// be a tax paid by every test method in the codebase.
  class TestBase : public ::testing::Test {
  public:
  static bool checkSingletonQuiescensce();
  ~TestBase() override;
  };
  -// Templatized version of TestBase.
  +// Templatized version of TestBase. See above notes.
  template <class T> class TestBaseWithParam : public ::testing::TestWithParam<T> {
  public:
  ~TestBaseWithParam() { TestBase::checkSingletonQuiescensce(); }
  