Disable Light Client model-based tests by default

[thanethomson]

Closes #968

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Added entry in .changelog/

[codecov-commenter]

Codecov Report

Merging #973 (eca466c) into master (80559bd) will decrease coverage by 0.0%.
The diff coverage is 90.1%.

@@           Coverage Diff            @@
##           master    #973     +/-   ##
========================================
- Coverage    72.1%   72.1%   -0.1%     
========================================
  Files         203     203             
  Lines       16577   16585      +8     
========================================
+ Hits        11961   11965      +4     
- Misses       4616    4620      +4     

Impacted Files	Coverage Δ
light-client/tests/model_based.rs	90.1% <90.1%> (+<0.1%)	⬆️
tendermint/src/signature.rs	56.6% <0.0%> (-4.1%)	⬇️
light-client/src/types.rs	29.2% <0.0%> (-0.3%)	⬇️
abci/src/server.rs	10.5% <0.0%> (+<0.1%)	⬆️
testgen/src/header.rs	80.7% <0.0%> (+0.6%)	⬆️
testgen/src/vote.rs	85.5% <0.0%> (+0.8%)	⬆️
tendermint/src/block/commit_sig.rs	78.7% <0.0%> (+2.1%)	⬆️

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[xla]

Besides the one comment, this is looking good.

🏢 ☺️ 🈴 🐝

[xla]

This change seems unrelated to the premise of this PR, could you contextualise what constitutes this change?

[thanethomson]

It's a prerequisite for being able to automatically generate changelog entries from the CLI using unclog, as per https://github.com/informalsystems/unclog#adding-a-new-unreleased-entry

I could've probably made the change in a separate PR, but it's so small, and I had to do it anyways, that I thought I'd just add it in here.