Add unittest

Add unittest

[   18s] + ./base64ct
[   18s]
[   18s] running 10 tests
[   18s] test decoder::tests::decode_padded ... ok
[   18s] test decoder::tests::decode_multiline_padded ... ok
[   18s] test decoder::tests::decode_unpadded ... ok
[   18s] test decoder::tests::read_multiline_padded ... ok
[   18s] test decoder::tests::decode_multiline_unpadded ... ok
[   18s] test encoder::tests::encode_multiline_padded ... ok
[   18s] test encoder::tests::encode_multiline_unpadded ... ok
[   18s] test encoder::tests::encode_unpadded ... ok
[   18s] test encoder::tests::encode_padded ... ok
[   18s] test encoder::tests::no_trailing_newline_when_aligned ... ok
[   18s]
[   18s] test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.02s