PrefixedMessage = function () {
  this.prefix = '';
  this.param = 'prefixedMessage'; // Param to use in querystrings
}
Supports pseudo-"namespacing" for window-posted messages for a given test
by generating and using a unique prefix that gets wrapped into message
objects. This makes it more feasible to have multiple tests that use
`window.postMessage` in a single test file. Basically, make it possible
for the each test to listen for only the messages that are pertinent to it.

'Prefix' not an elegant term to use here but this models itself after
PrefixedLocalStorage.

PrefixedMessageTest: Instantiate in testharness.js tests to generate
  a new unique-ish prefix that can be used by other test support files
PrefixedMessageResource: Instantiate in supporting test resource
  files to use/share a prefix generated by a test.