Make HMMBase optional in the test suite

[gdalle]

• Put HMMBase-dependent code in an extension of the internal package HMMTest
• Split CI and test suite depending on whether we want to load HMMBase
• Make some test sequences a little longer
• Remove all @inbounds

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.07%. Comparing base (9802bad) to head (3bb007d).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/inference/baum_welch.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #111   +/-   ##
=======================================
  Coverage   90.07%   90.07%           
=======================================
  Files          17       17           
  Lines         504      504           
=======================================
  Hits          454      454           
  Misses         50       50           

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