testgen: Compute last_block_id hash when generating a light chain

[romac]

Close #744
Close #745

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG.md

[codecov-io]

Codecov Report

Merging #746 (57b0a6d) into master (4225d7f) will increase coverage by 0.0%.
The diff coverage is 77.1%.

@@          Coverage Diff           @@
##           master    #746   +/-   ##
======================================
  Coverage    43.2%   43.3%           
======================================
  Files         203     203           
  Lines       13118   13169   +51     
  Branches     3456    3483   +27     
======================================
+ Hits         5673    5705   +32     
- Misses       7087    7104   +17     
- Partials      358     360    +2     

Impacted Files	Coverage Δ
testgen/src/light_block.rs	79.8% <0.0%> (-1.9%)	⬇️
testgen/src/header.rs	75.5% <54.5%> (-2.1%)	⬇️
testgen/src/light_chain.rs	93.9% <88.3%> (+1.3%)	⬆️
tendermint/src/hash.rs	57.2% <0.0%> (-6.2%)	⬇️
testgen/src/helpers.rs	78.3% <0.0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4225d7f...57b0a6d. Read the comment docs.

[Shivani912]

Thank you for doing this @romac ! ❤️

[Shivani912]

Approved 👍