TST: increase test coverage of stats module.
BUG: fix incorrect output from stats.tsem.  Add tests for trimmed stats funcs.
TST: fix non-running tests for stats.itemfreq.

TestCase subclasses don't work with test generators.
TST: fix non-running test for stats.mannwhitneyu.
Merge pull request #2913 from rgommers/stats-coverage

stats.py fixes and test coverage