Add a MerkleHashOrchard::random function under the test-dependencies feature.

[nuttycom]

No description provided.

[str4d]

utACK. This can I think generate MerkleHashOrchard values that aren't technically possible to produce normally, but this is fine for testing.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 80.49%. Comparing base (1db9741) to head (7ef1fea).

Files	Patch %	Lines
src/tree.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #422      +/-   ##
==========================================
- Coverage   80.54%   80.49%   -0.06%     
==========================================
  Files          30       30              
  Lines        3156     3158       +2     
==========================================
  Hits         2542     2542              
- Misses        614      616       +2     

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