test: slow down CandidateFinder test to avoid clock race

[rvagg]

I'm attempting to deal with the flaky CandidateFinder test, which I can't replicate locally. I think the main problem here is that we tick the clock inside the test 1 minute per candidate, but the mock sleeps for 1 second per candidate.

The reasons this works most of the time is that the goroutine startup is fast enough to get started and get to the clock.Sleep before the timer gets through its ticks. The mock clock will do an actual 1ms sleep for each Add to let the goroutine scheduler catch up, so for most cases we get enough breathing room to get at least one clock.Add in after the clock.Sleep.

So there's two things going on in this PR:

1. Change test's clock tick to 1s
2. Add a channel into the mock that lets the test observe the incoming requests; for the async request it can wait until the request has come in before moving on to ticking the clock

[codecov-commenter]

Codecov Report

Merging #304 (02fc686) into main (6fe2efe) will increase coverage by 0.49%.
The diff coverage is 60.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #304      +/-   ##
==========================================
+ Coverage   72.47%   72.96%   +0.49%     
==========================================
  Files          68       68              
  Lines        6242     6248       +6     
==========================================
+ Hits         4524     4559      +35     
+ Misses       1465     1447      -18     
+ Partials      253      242      -11     

Impacted Files	Coverage Δ
pkg/internal/mockindexer/mockindexer.go	72.91% <60.86%> (-5.98%)	⬇️

... and 6 files with indirect coverage changes

[rvagg]

:shakesfist: https://github.com/filecoin-project/lassie/actions/runs/5208640304/jobs/9397456670?pr=304

The second time I've attempted to fix this test but had the test fail when I've put up a PR

[rvagg]

Combined with the timeout extension in #294 and I've rerun CI a bunch of times and have not seen the failure again.

[hannahhoward]

LGTMing for now