TST: test_fast.sh and test_multi.sh should skip network tests
Merge pull request #5265 from cpcloud/update-test-fast-script-no-network

TST: test_fast.sh and test_multi.sh should skip network tests
EHN/FIX: Add na_position parameter to DataFrame.sort. Fixes GH3917
EHN: DataFrame can now sort a single column or label by all three parameters:
     kind, ascending and na_position.

Added _nargsort, and extension of np.argsort that has ascending and na_position
parameters.
Rewrite _nargsort
Skip mergesort test if `np.argsort` raises TypeError on any mergesort.
Use `assert_frame_equivalent` to test if DataFrames are equal (ignoring NaNs).