Add unittest

Add unittest
[   12s] + ./lebe
[   12s]
[   12s] running 15 tests
[   12s] test cmp_read_be_slice ... ok
[   12s] test cmp_read_be_u16 ... ok
[   12s] test cmp_read_le_f32 ... ok
[   12s] test cmp_read_le_u16 ... ok
[   12s] test cmp_write_le_slice ... ok
[   12s] test cmp_write_le_u32 ... ok
[   12s] test into_be_f64 ... ok
[   12s] test cmp_write_le_slice_u64 ... ok
[   12s] test into_be_i16 ... ok
[   12s] test into_be_u32 ... ok
[   12s] test make_be_f64 ... ok
[   12s] test make_be_u32_slice ... ok
[   12s] test make_le_i64_slice ... ok
[   12s] test make_le_u32_slice ... ok
[   12s] test make_le_u16_slice ... ok
[   12s]
[   12s] test result: ok. 15 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.02s