MDL-37086 phpunit: use alt reset method.

When code executed in test cases is using/preventing
database transactions, phpunit itself cannot
use transactions for reseting between cases. But we
can tell it about, so some alternative (and slower)
method for reseting will be used.

message/tests/externallib_test.php

@@ -62,6 +62,8 @@ public function test_send_instant_messages() {
         global $DB, $USER, $CFG;
 
         $this->resetAfterTest(true);
+        // Transactions used in tests, tell phpunit use alternative reset method.
+        $this->preventResetByRollback();
 
         // Turn off all message processors (so nothing is really sent)
         require_once($CFG->dirroot . '/message/lib.php');