Update rstar tests

[sethaxen]

TuringLang/MCMCDiagnosticTools.jl#52 updated rstar to by default split the chains in two to test for within-chain convergence. This causes the final rstar test to fail, so this PR updates the test to explicitly use no splitting.

[codecov]

Codecov Report

Base: 83.67% // Head: 83.76% // Increases project coverage by +0.09% 🎉

Coverage data is based on head (7587cb1) compared to base (5f59160).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
+ Coverage   83.67%   83.76%   +0.09%     
==========================================
  Files          19       19              
  Lines        1035     1035              
==========================================
+ Hits          866      867       +1     
+ Misses        169      168       -1     

Impacted Files	Coverage Δ
src/MCMCChains.jl	100.00% <ø> (+50.00%)	⬆️
src/discretediag.jl	100.00% <ø> (ø)
src/ess.jl	100.00% <100.00%> (ø)
src/gelmandiag.jl	100.00% <100.00%> (ø)
src/rstar.jl	100.00% <100.00%> (ø)
src/utils.jl	95.29% <100.00%> (+0.05%)	⬆️

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