drop unitt2 and use tox in 'make test' target

[cgoldberg]

since #755 removed the dependency on unittest2,
this PR changes the make test target so it calls tox instead of unit2

[codecov]

Codecov Report

Merging #772 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #772   +/-   ##
=======================================
  Coverage   65.13%   65.13%           
=======================================
  Files          14       14           
  Lines        1417     1417           
  Branches      222      222           
=======================================
  Hits          923      923           
  Misses        443      443           
  Partials       51       51

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 46628ed...ccc61c1. Read the comment docs.

[aldenpeterson-wf]

+1