Add a basic test case for the cache

This very basic test ensures that the useCache method works as
expected for simple scenarios. This test case fails without commit
a8795974 and then also triggers the
warning that was introduced in bc2ddb54

_test/tests/inc/cache_use.test.php

+<?php
+
+/**
+ * Class cache_use_test
+ *
+ * Tests if caching can actually be used
+ */
+class cache_use_test extends DokuWikiTest {
+    /** @var cache_renderer $cache */
+    private $cache;
+
+    function setUp() {
+        global $ID;
+        parent::setUp();
+
+        $ID = 'cached';
+        $file = wikiFN($ID);
+
+        saveWikiText($ID, 'Content', 'Created');
+        // set the modification time a second in the past in order to ensure that the cache is newer than the page
+        touch($file, time()-1);
+
+        # Create cache. Note that the metadata cache is used as the xhtml cache triggers metadata rendering
+        $this->cache = new cache_renderer($ID, $file, 'metadata');
+        $this->cache->storeCache('Test');
+    }
+
+    function test_use() {
+        $this->assertTrue($this->cache->useCache());
+    }
+
+
+    function test_purge() {
+        $this->assertFalse($this->cache->useCache(array('purge' => true)));
+    }
+}
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