M2: DDL and DML functional tests

From some chats:

http://docs.google.com/View?docid=dgzf39jp_7tmcr5jc5 :

• table prefix is separated so it's easy to create moodle_database instances with different prefixes.
• able to test execute test agains multiple DBs by:
  1. Having some interface in admin to define them
  2. Make tests happen against all them
  3. If no alternate DBs are defined, tets default to current one (config.php one)
• will go to dml/simpletest and ddl/simpletest

HQ chat:

• able to be executed with one CLI script.
• able to generate one cool page with all test results.
• able to report by email about problems.

Ciao