ENH: Add regression results to Phillips-Perron test

[bashtage]

Add regression results

closes #395

[codecov]

Codecov Report

Merging #398 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #398   +/-   ##
=======================================
  Coverage   99.36%   99.36%           
=======================================
  Files          72       72           
  Lines       13274    13285   +11     
  Branches     1131     1131           
=======================================
+ Hits        13190    13201   +11     
  Misses         28       28           
  Partials       56       56           

Impacted Files	Coverage Δ
arch/tests/unitroot/test_unitroot.py	100.00% <100.00%> (ø)
arch/unitroot/unitroot.py	98.89% <100.00%> (+<0.01%)	⬆️

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[coveralls]

Coverage increased (+0.0002%) to 99.788% when pulling a3f865b on reg-results-pp into 4cb68db on master.