Revert TEMP_FOLDER change from ccc8dd061c

It obscures the fact that you're in a test session
by hiding it away in system folders. Having it in assets/
is confusing enough if your webroot isn't writeable.

/cc @wilr

code/TestSessionEnvironment.php

@@ -72,10 +72,6 @@ class TestSessionEnvironment extends Object {
 			$path = Director::getAbsFile($this->config()->test_state_file);	
 		}
 
-		if(!is_writable(dirname($path))) {
-			$path = str_replace(Director::baseFolder(), TEMP_FOLDER, $path);
-		}
-
 		return $path;
 	}