[tests] Improved code coverage report

Made a test which loads all classes; this way code coverage is more relevant.
[tests] Big structure changes

Replaced UnitTestHelper by ClassTester: allows testing classes independently of their parent classes and less duplication.

Created a directory per class instead of a data directory per class:
[tests] + AjaxQueryPart
[tests/Context] Exceptions are automaticaly loaded
[tests] + AjaxQuery