Added unit test for "overwrite file on rename/move"

Also fixed "rename" unit test that was ready the result out of the wrong
file.

tests/lib/files/storage/storage.php

@@ -139,6 +139,12 @@ abstract class Storage extends \PHPUnit_Framework_TestCase {
 		$this->instance->rename('/source.txt', '/target2.txt');
 		$this->assertTrue($this->instance->file_exists('/target2.txt'));
 		$this->assertFalse($this->instance->file_exists('/source.txt'));
+		$this->assertEquals(file_get_contents($textFile), $this->instance->file_get_contents('/target2.txt'));
+
+		// move to overwrite
+		$this->instance->rename('/target2.txt', '/target.txt');
+		$this->assertTrue($this->instance->file_exists('/target.txt'));
+		$this->assertFalse($this->instance->file_exists('/target2.txt'));
 		$this->assertEquals(file_get_contents($textFile), $this->instance->file_get_contents('/target.txt'));
 	}