def flatten_tests(test_classes):
    """
    >>> test_classes = {x: [x] for x in range(5)}
    >>> flatten_tests(test_classes)
    [(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)]
    >>> test_classes = {x: [x + 1, x + 2] for x in range(2)}
    >>> flatten_tests(test_classes)
    [(0, 1), (0, 2), (1, 2), (1, 3)]
    """
    tests = []
    for class_name, test_names in test_classes.items():
        tests += [(class_name, test_name) for test_name in test_names]
    return tests