php: make the test suite failures fatal

So far the failure of some test would have not reported a non-zero
return value by run-tests.php. Since now all the PHP tests pass, we can
ask for failures to be fatal, by exporting REPORT_EXIT_STATUS=1 for
run-tests.php.
php: move tests under a tests subdirectory

This eases fixes the classname in the junit output of run-tests.php,
which now can be properly handled by junit XML consumers.