Fix missing hasher parameter for supervisor test

[thanethomson]

Minor fix. This came up in CI as well as while trying to test out the commands in #790.

• 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 #793 (419115c) into master (4752a88) will increase coverage by 2.0%.
The diff coverage is 84.8%.

@@           Coverage Diff            @@
##           master    #793     +/-   ##
========================================
+ Coverage    53.6%   55.7%   +2.0%     
========================================
  Files         197     198      +1     
  Lines       13682   14060    +378     
  Branches     3482    3692    +210     
========================================
+ Hits         7341    7834    +493     
+ Misses       5942    5894     -48     
+ Partials      399     332     -67     

Impacted Files	Coverage Δ
light-client/src/builder/light_client.rs	0.0% <0.0%> (ø)
light-client/src/errors.rs	9.6% <0.0%> (+1.7%)	⬆️
light-client/src/store.rs	40.6% <0.0%> (ø)
light-client/src/tests.rs	62.0% <ø> (+6.1%)	⬆️
testgen/src/light_chain.rs	86.0% <71.4%> (-7.9%)	⬇️
light-client/tests/backward.rs	79.1% <79.1%> (ø)
light-client/src/light_client.rs	75.0% <88.8%> (+18.3%)	⬆️
light-client/src/supervisor.rs	80.2% <92.2%> (+51.5%)	⬆️
light-client/src/utils/std_ext.rs	62.5% <100.0%> (+31.2%)	⬆️
light-client/tests/light_client.rs	87.0% <100.0%> (+1.3%)	⬆️
... and 75 more

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