MDL-71731 phpunit: isolate tests requiring lots of peak mem

It's mostly addDocuments(), used by test_add_document_batch_large()
with 100 big documents what requires a lot of ram
although apparently, it's freed (partially) once ended.
Just the peak usage remains high. In any case, isolating that
test to avoid the non-freed side of it to consume too
much memory for the rest. (We allow "only" 2GB).

Still, I think that there is a good work about to detect which
tests are leaking / consuming too much, I'd bet there are
a bunch running completely out of control.

search/engine/solr/tests/engine_test.php

@@ -50,6 +50,8 @@ require_once($CFG->dirroot . '/search/engine/solr/tests/fixtures/testable_engine
  * @category    phpunit
  * @copyright   2015 David Monllao {@link http://www.davidmonllao.com}
  * @license     http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ *
+ * @runTestsInSeparateProcesses
  */
 class search_solr_engine_testcase extends advanced_testcase {