ajax_validation_test breaks when manually tested

Problem/Motivation

While converting ElementValidationTest into a FunctionalJavascript test ( see the parent issue )
The good news is that with mink we can start to test the associated javascript - and the test becomes closer to what manual testing does.

The bad news is manual testing of the test page is currently broken as it triggers an infinite loop of AJAX requests and responses.

I want to split this off as a side issue ... so we leave a audit trail of what was done and why.

Steps to reproduce

// enable 2 test modules

1. drush en ajax_test ajax_forms_test
2. visit /ajax_validation_test
3. enter "some dumb text" into first box
4. enter 1234 into field below.

// watch as the form goes haywire ... triggering an endless loop of ajax requests!

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet