message: Add methodForTest() convention to the style guide.
diff: exceptions such as `main()` functions. When code cannot be placed inside the
  Envoy namespace there should be a comment of the form `// NOLINT(namespace-envoy)` at
  the top of the file.
  +* If a method that must be defined outside the `test` directory is intended to be called only
  + from test code then it should have a name that ends in `ForTest()` such as `aMethodForTest()`.
  + In most cases tests can and should be structured so this is not necessary.
  * There are probably a few other things missing from this list. We will add them as they
  are brought to our attention.
  