test: Cli: add mempool tests

[brdji]

Related Issues

Mempool cli commands do not have any test coverage. This PR adds basic tests covering (almost) every cli command available for the mempool system.

Proposed Changes

Add a test for every mempool cli command.
Most of the tests perform simple "sanity" checks to see whether the cli command has been performed successfully. Even though they don't check for anything really substantial, having even basic tests around is invaluable whenever a new cli change is introduced.

Checklist

Before you mark the PR ready for review, please make sure that:

• All commits have a clear commit message.
• The PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, INTERFACE BREAKING CHANGE, CONSENSUS BREAKING, build, chore, ci, docs,perf, refactor, revert, style, test
  • area: api, chain, state, vm, data transfer, market, mempool, message, block production, multisig, networking, paychan, proving, sealing, wallet, deps
• This PR has tests for new functionality or change in behaviour
• If new user-facing features are introduced, clear usage guidelines and / or documentation updates should be included in https://lotus.filecoin.io or Discussion Tutorials.
• CI is green

[codecov]

Codecov Report

Merging #8162 (a54d2fd) into master (5d416de) will increase coverage by 0.23%.
The diff coverage is 56.66%.

@@            Coverage Diff             @@
##           master    #8162      +/-   ##
==========================================
+ Coverage   39.91%   40.15%   +0.23%     
==========================================
  Files         666      666              
  Lines       72554    72561       +7     
==========================================
+ Hits        28963    29135     +172     
+ Misses      38553    38321     -232     
- Partials     5038     5105      +67     

Impacted Files	Coverage Δ
cli/mpool.go	58.82% <56.66%> (+58.82%)	⬆️
markets/loggers/loggers.go	88.88% <0.00%> (-11.12%)	⬇️
chain/actors/builtin/miner/diff.go	48.52% <0.00%> (-10.30%)	⬇️
cli/util.go	70.83% <0.00%> (-8.34%)	⬇️
chain/exchange/peer_tracker.go	66.66% <0.00%> (-4.31%)	⬇️
storage/wdpost_sched.go	77.45% <0.00%> (-3.93%)	⬇️
chain/exchange/client.go	49.15% <0.00%> (-3.82%)	⬇️
chain/stmgr/call.go	67.87% <0.00%> (-3.64%)	⬇️
chain/vm/vm.go	61.63% <0.00%> (-1.07%)	⬇️
extern/storage-sealing/fsm.go	56.44% <0.00%> (-1.05%)	⬇️
... and 12 more

---

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 5d416de...a54d2fd. Read the comment docs.