Test: use xdist
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 
```