Tweak sample counts and other details in tests

[mhauru]

This PR makes a number of small changes to the tests

• Lowers the sample counts of many tests.
• Adds a few comments, including some TODO notes saying we should add an explanatory comment.
• Uses StableRNGs more. Still not using them everywhere, but I added them in many places where stability matters.
• Removes unnecessary calls to Random.seed!. Every @testset resets the seed anyway.
• Removes some dead or commented out tests that weren't doing anything.
• Adds verbose = true to many of the top-level @testsets, to get some timing numbers even when tests pass.
• Adds info prints in the beginning of some of the longer-running testsets, saying which test is starting.
• Adds some more subtestsets to some of the longer-running ones, to get more granular info about what fails.
• Fixes one commented-out test to actually do something.

This started with me thinking that since we do a lot of samples in many tests, and our test suite is quite slow, clearly we should reduce the sample counts if we can. What I learned in the process is that most of the tests that do sampling are negligble compared to the tests that use particle methods. Anything involving PG/CSMC tends to take 10-100 times longer than comparable tests with other samplers.

In the end I've tried to adjust iteration counts only when either the count truly doesn't matter (we are not checking stats of the chain), or when doing so actually has a noticable, though not necessarily huge, impact on run time. I may have slipped a few times, and reduced something that didn' really make a difference. Also, when reducing iteration counts I tried to check that they pass if I pick a few different seeds, to make sure I'm not making them very brittle to future changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.46%. Comparing base (c0a4ee9) to head (66be15d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2433      +/-   ##
==========================================
- Coverage   44.72%   44.46%   -0.26%     
==========================================
  Files          22       22              
  Lines        1554     1554              
==========================================
- Hits          695      691       -4     
- Misses        859      863       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[coveralls]

Pull Request Test Coverage Report for Build 12261740370

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.3%) to 37.831%

---

Files with Coverage Reduction	New Missed Lines	%
src/mcmc/Inference.jl	2	52.44%
src/mcmc/abstractmcmc.jl	2	92.86%

Totals
Change from base Build 12110228665:	-0.3%
Covered Lines:	586
Relevant Lines:	1549

---

💛 - Coveralls

[mhauru]

I'm hoping CI will now pass, except for Mooncake stack overflows. See updated OP for what this PR does.