TST: speed up some very slow tests

Minimize alignment combinations to useful set with SSE, might need to
be revisited if we add AVX support.
Move large data test_zeros test to slow and drop the extremely slow
count_nonzero() call.
Rename gen_alignment_data to _gen_alignment_data (private) to reserve
right to change it, e.g. add min_size.
TST: delay error message building to failures in assert_almost_equal

improves test performance by about 15%
Merge pull request #3790 from juliantaylor/test-speedup

TST: speed up some very slow tests