Add test cases for `Pad()` for edge cases

fa2c31dec2fbcb7a6d54e8f1d2e2730baadf8120 fixed an issue in which the padding method was failing where size of input block % 128 was being more than 112. This was resulting in wrong padding and thus incorrect hashes. It's likely there are similar other edge cases which are not tested. We need to add test cases for that.

Start by inspecting commit fa2c31dec2fbcb7a6d54e8f1d2e2730baadf8120 and the Pad() implementation to find the existing padding tests. Add cases covering the reported block-size boundary and similar edge cases, then run the Go test suite and verify that the resulting hashes are correct.

Written by the indexing model from the issue text.