Test: use xdist

[jreback]

on top of #15369
xref #15341

pytest pandas --skip-slow --skip-network -m "not single" -n 4

(pandas) bash-3.2$ ./test_fast.sh 
==================================================================================================== test session starts ====================================================================================================
platform darwin -- Python 3.5.2, pytest-3.0.5, py-1.4.31, pluggy-0.4.0
rootdir: /Users/jreback/pandas, inifile: setup.cfg
plugins: cov-2.4.0, xdist-1.15.0
gw0 [11470] / gw1 [11470] / gw2 [11470] / gw3 [11470]
scheduling tests via LoadScheduling
.....s...ss......s...s....s....s.....s...s...s.s......................s..............s......s...s................s......s...ss.....s......................................................................................................................................s..s.s.s......................................................................................................................................................................................s.```

=============== 11046 passed, 424 skipped in 170.96 seconds

[codecov-io]

Codecov Report

Merging #15369 into master will decrease coverage by -0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15369      +/-   ##
==========================================
- Coverage   90.44%   90.42%   -0.03%     
==========================================
  Files         134      134              
  Lines       49357    49357              
==========================================
- Hits        44642    44632      -10     
- Misses       4715     4725      +10

Impacted Files	Coverage Δ
pandas/util/testing.py	82.44% <ø> (-0.94%)	❌
pandas/tseries/offsets.py	97.13% <ø> (-0.13%)	❌
pandas/core/frame.py	97.87% <ø> (ø)	✅
pandas/util/terminal.py	47.82% <ø> (+2.89%)	✅

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[jreback]

merged this. cuts about 1 hr off of total time. https://travis-ci.org/jreback/pandas/builds/200891319

this is using -n 2; -n 4 helps when you actually have that many cores, travis only has 2 (by default).