BIP380: make specs consistent about hardened indicators

[quapka]

Me and my students noticed a discrepancy in the test vectors and specification regarding the allowed characters denoting hardened nodes. The BIP380 specs allows all three h, ' and H, in particular the specs read:

In the above specification, the hardened indicator h may be replaced with alternative
hardened indicators of H or '.

However, the following is then an invalid test vectors:

[deadbeef/0H/0H/0H]0260b2003c386519fc9eadf2b5cf124dd8eea4c4e68d5e154050a9346ea98ce600

Thus, I've added new test vectors allowing H and removed the inconsistent one.

However, I am far from claiming that this is the correct way how to handle this. I've checked that people are using the H, but I got no (meaningful or relevant) hits when searching for the exact test vector. I didn't see the option to create issues, thus I've continued by proposing the, what seems to be, reasonable changes.

[murchandamus]

cc: @achow101