Warning when running tests without flush=1

This is now the default setting for both "sake" and "phpunit"
runs, because of performance reasons (every manifest flush takes
multiple seconds). On the other hand, we want to make errors
like missing classes more obvious to developers.
See discussion in #620

dev/TestRunner.php

@@ -106,6 +106,14 @@ function init() {
 		if(!PhpUnitWrapper::has_php_unit()) {
 			die("Please install PHPUnit using pear");
 		}
+
+		if(!isset($_GET['flush']) || !$_GET['flush']) {
+			Debug::message(
+				"WARNING: Manifest not flushed. " .
+				"Add flush=1 as an argument to discover new classes or files.\n",
+				false
+			);
+		}
 	}
 
 	public function Link() {

tests/bootstrap.php

@@ -62,4 +62,12 @@
 TestRunner::use_test_manifest();
 
 // Remove the error handler so that PHPUnit can add its own
-restore_error_handler();
+restore_error_handler();
+
+if(!isset($_GET['flush']) || !$_GET['flush']) {
+	Debug::message(
+		"WARNING: Manifest not flushed. " .
+		"Add flush=1 as an argument to discover new classes or files.\n",
+		false
+	);
+}