cleaned up grouping tests to take advantage of new T and Val

[codecov[bot]]

Codecov Report

Merging #251 (35d565d) into master (13432c2) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #251   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files          27       27           
  Lines        4380     4380           
  Branches      764      760    -4     
=======================================
  Hits         4304     4304           
  Misses         48       48           
  Partials       28       28           

Impacted Files	Coverage Δ
glom/grouping.py	98.72% <100.00%> (ø)
glom/test/test_grouping.py	100.00% <100.00%> (ø)

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[mahmoud]

Much cleaner, nice!