Fix testing

[petrelharp]

I discovered some errors in the testing setup while mucking around with #328, and have pulled those out. These changes depend on #313; ignore those commits. I've run all the stats tests with p=1.0 to make sure things are good. Note that there were no errors with the actual implementations, only with two of our "naive" implementations.

[codecov]

Codecov Report

Merging #330 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #330      +/-   ##
==========================================
- Coverage   86.47%   86.47%   -0.01%     
==========================================
  Files          20       20              
  Lines       14035    14034       -1     
  Branches     2751     2751              
==========================================
- Hits        12137    12136       -1     
  Misses        979      979              
  Partials      919      919

Flag	Coverage Δ
#c_tests	87.55% <100%> (-0.01%)	⬇️
#python_c_tests	90.3% <100%> (-0.01%)	⬇️
#python_tests	99.23% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
python/tskit/trees.py	98.66% <100%> (-0.01%)	⬇️

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[petrelharp]

Oh, this PR actually closes #297.

[jeromekelleher]

This is great, thanks @petrelharp. I'll start merging stuff in now (need to do a bit of rebasing first, so leave it with me).