Add TryteString to bytes test cases

[mlouielu]

As for issue #62, this is the test cases for converting
TryteString to bytes, and bytes to TryteString.

(CI dead is correct)

[todofixthis]

Hey @mlouielu thank you for working on this! I've asked some of the core devs (who are better at maths than I am 😸) to have a look at this.

I had a few questions about these tests:

• What should TryteString('NOP').as_bytes() return? (negative value)
• I see that TryteString.from_bytes('', length=1) returns TryteString('9'). But, TryteString('').as_bytes(length=1) raises an IndexError; wouldn't it be more consistent if it returned b'\x00'?
• I think the tests cover this, but help me to understand because I still don't get it 😇 :
  • Converting 27 trits to bytes: 27 % 5 != 0, so how is this conversion handled?

[todofixthis]

Hey @mlouielu thanks again for your help with this feature! I'm working with a few individuals to try to nail down requirements for this codec — I feel like there are a number of edge cases that the existing implementations are not able to cover (https://github.com/vbakke/trytes/ has an excellent write-up of many of these issues).

In the meantime, I've put #62 on hold until we can work out all of the details.