MDL-32149 PHPUnit test support - part 2

Includes:
* constants refactoring
* reworked db table init
* support for $CFG->debug = -1
* functional DB tests
* fixed $DB->get_indexes() to not throw exceptions when table does not exist
* fix handling of user passwords in test db
* add debug info to exception messages
* removed unnecessary PHP debug errors from mathslib
* fixed @error suppression in get_string
* fixed PHPUnit error handler setup
* added timezone info to default install

lib/pluginlib.php

@@ -479,7 +479,7 @@ class plugin_manager {
 
             'tool' => array(
                 'bloglevelupgrade', 'capability', 'customlang', 'dbtransfer', 'generator',
-                'health', 'innodb', 'langimport', 'multilangupgrade', 'profiling',
+                'health', 'innodb', 'langimport', 'multilangupgrade', 'phpunit', 'profiling',
                 'qeupgradehelper', 'replace', 'spamcleaner', 'timezoneimport', 'unittest',
                 'uploaduser', 'unsuproles', 'xmldb'
             ),