fix(tests): Fix fopen in mock returning null which is not a valid return

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

Author: come-nc

tests/lib/Files/ViewTest.php

@@ -1183,6 +1183,9 @@ private function doTestCopyRenameFail($operation) {
 		$storage2->method('writeStream')
 			->willThrowException(new GenericFileException('Failed to copy stream'));
 
+		$storage2->method('fopen')
+			->willReturn(false);
+
 		$storage1->mkdir('sub');
 		$storage1->file_put_contents('foo.txt', '0123456789ABCDEFGH');
 		$storage1->mkdir('dirtomove');