Changed phpunit Makefile to require 'make destructive' to run the test suites as...

Changed phpunit Makefile to require 'make destructive' to run the test suites as a temporary safety check.

Looks like somebody's snuck a bunch of destructive tests into the default PHPUnit test suite;
this is unsafe and could cause admins and developers to damage their wikis contents accidentally,
not realizing that their main page will be deleted etc.

Tests should be split up into those which alter the live database and those that don't; only
those which are non-destructive should run unless explicitly requested.