Check that two files are not the same by comparing the fid and filepath.
$file1: File object to compare.
$file2: File object to compare.
function assertDifferentFile($file1, $file2) {
$this
->assertNotEqual($file1->fid, $file2->fid, format_string('Files have different ids: %file1 != %file2.', array(
'%file1' => $file1->fid,
'%file2' => $file2->fid,
)), 'Different file');
$this
->assertNotEqual($file1->uri, $file2->uri, format_string('Files have different paths: %file1 != %file2.', array(
'%file1' => $file1->uri,
'%file2' => $file2->uri,
)), 'Different file');
}