test: remaining incentive module unit tests

[toteki]

closes: #2005
progress: #1990

Note: What remains in #1990 is the leverage-side tests (basically hooks_test.go but run from the leverage module during its integration tests) and some edge cases.

[codecov]

Codecov Report

Merging #2061 (400b08e) into main (7f05ad4) will decrease coverage by 0.57%.
The diff coverage is 78.29%.

@@            Coverage Diff             @@
##             main    #2061      +/-   ##
==========================================
- Coverage   75.38%   74.82%   -0.57%     
==========================================
  Files         100      126      +26     
  Lines        8025     9632    +1607     
==========================================
+ Hits         6050     7207    +1157     
- Misses       1589     1948     +359     
- Partials      386      477      +91     

Impacted Files	Coverage Δ
ante/spam_prevention.go	75.92% <ø> (ø)
util/store/unmarshal.go	0.00% <0.00%> (ø)
x/incentive/codec.go	47.82% <ø> (+9.89%)	⬆️
x/incentive/keeper/invariants.go	0.00% <0.00%> (ø)
x/incentive/keeper/keeper.go	64.28% <ø> (ø)
x/leverage/keeper/errors.go	100.00% <ø> (ø)
x/leverage/keeper/exchange_rate.go	72.72% <ø> (ø)
x/leverage/keeper/genesis.go	87.23% <ø> (ø)
x/leverage/keeper/grpc_query.go	67.21% <ø> (ø)
x/leverage/keeper/incentive.go	0.00% <0.00%> (ø)
... and 69 more

... and 23 files with indirect coverage changes

[robert-zaremba]

start of the review

[robert-zaremba]

We should have more tests which can interface the flow... eg: bond --> unbond partially --> check if rewards were paid --> more people bond --> wait few blocks --> see if rewards are distributed correctly. I can create few tasks with specific flows we should test.

NOTE: Would be good to follow the KeeperBuilder pattern (but this is not blocking the PR).