Add unittest

Add unittest
[   28s] + ./unicode_segmentation
[   28s]
[   28s] running 12 tests
[   28s] test grapheme::test_grapheme_cursor_chunk_start_require_precontext ... ok
[   28s] test grapheme::test_grapheme_cursor_prev_boundary ... ok
[   28s] test grapheme::test_grapheme_cursor_prev_boundary_chunk_start ... ok
[   28s] test grapheme::test_grapheme_cursor_ris_precontext ... ok
[   28s] test test::test_graphemes ... ok
[   28s] test test::quickcheck_forward_reverse_graphemes_extended ... ok
[   28s] test test::quickcheck_forward_reverse_graphemes_legacy ... ok
[   28s] test test::quickcheck_join_graphemes ... ok
[   28s] test test::test_sentences ... ok
[   28s] test test::quickcheck_join_words ... ok
[   28s] test test::quickcheck_forward_reverse_words ... ok
[   28s] test test::test_words ... ok
[   28s]
[   28s] test result: ok. 12 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.03s