This will test the filepath for a destination based on passed flags and whether or not the file exists.
If a file exists, file_destination($destination, $replace) will either return:
If the file doesn't currently exist, then it will simply return the filepath.
function testFileDestination() {
// First test for non-existent file.
$destination = 'core/misc/xyz.txt';
$path = file_destination($destination, FILE_EXISTS_REPLACE);
$this
->assertEqual($path, $destination, 'Non-existing filepath destination is correct with FILE_EXISTS_REPLACE.', 'File');
$path = file_destination($destination, FILE_EXISTS_RENAME);
$this
->assertEqual($path, $destination, 'Non-existing filepath destination is correct with FILE_EXISTS_RENAME.', 'File');
$path = file_destination($destination, FILE_EXISTS_ERROR);
$this
->assertEqual($path, $destination, 'Non-existing filepath destination is correct with FILE_EXISTS_ERROR.', 'File');
$destination = 'core/misc/druplicon.png';
$path = file_destination($destination, FILE_EXISTS_REPLACE);
$this
->assertEqual($path, $destination, 'Existing filepath destination remains the same with FILE_EXISTS_REPLACE.', 'File');
$path = file_destination($destination, FILE_EXISTS_RENAME);
$this
->assertNotEqual($path, $destination, 'A new filepath destination is created when filepath destination already exists with FILE_EXISTS_RENAME.', 'File');
$path = file_destination($destination, FILE_EXISTS_ERROR);
$this
->assertEqual($path, FALSE, 'An error is returned when filepath destination already exists with FILE_EXISTS_ERROR.', 'File');
}