added support for plugin unittests

	This patch adds support to include plugin tests in the
	DokuWiki testsuite. Plugin tests are located in a dedicated
	directory _test/within a plugin directory. The naming
	convention of the test files follows the one used in
	DokuWikis testsuite.

		<plugin>/_test/*.test.php 	-> single test
		<plugin>/_test/*.group.php  -> group test

	The plugin tests are accessible via the web interface
	of the test suite and via the cli interface. It is recommend
	to bundle plugin test in a plugin group test. The webinterface
	also allows to run all plugin tests at once.

	Test files must include:

		<dokuwiki>/_test/lib/unittest.php

	Example Test:

	require_once(DOKU_INC.'_test/lib/unittest.php');
	class plugin_test extends Doku_UnitTestCase {
		function test() {
			$this->assertEqual(1,1);
		}
	}

	Example Group Test:

	require_once(DOKU_INC.'_test/lib/unittest.php');
	class plugin_group_test extends Doku_GroupTest {
		function group_test() {
			$dir = dirname(__FILE__).'/';
			$this->GroupTest('plugin_grouptest');
			$this->addTestFile($dir . 'plugin.test1.php');
			$this->addTestFile($dir . 'plugin.test2.php');
			$this->addTestFile($dir . 'plugin.test3.php');
		}
	}

	At the moment unittest.php contains only two
	meta classes so plugins tests don't have to inherit
	from the simpletest classes.

	This patch should be treated as intermediate step to
	allow for plugin tests. The testsuite wasn't designed
	to include plugin tests. It should probably be refactored
	at a later point.