Allow users to run the DistArray tests from the installed package

This PR enables:

```
In [1]: import distarray

In [2]: distarray.test()
IPython.parallel client tests:
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Ran 327 tests in 14.949s

OK (skipped=10)

MPI client tests:
mpiexec -np 1 python -m unittest discover -c -s /Users/robertgrant/development/distarray : -np 4 /Users/robertgrant/developm
ent/distarray/distarray/apps/engine.py
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----------------------------------------------------------------------
Ran 327 tests in 3.444s

OK (skipped=12)

Engine-only MPI tests:
mpiexec -np 4 python -m unittest discover -cp paralleltest_*.py -s /Users/robertgrant/development/distarray
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----------------------------------------------------------------------
Ran 139 tests in 0.527s

OK (skipped=7)

----------------------------------------------------------------------
Ran 139 tests in 0.525s

OK (skipped=7)

----------------------------------------------------------------------
Ran 139 tests in 0.522s

OK (skipped=4)

----------------------------------------------------------------------
Ran 139 tests in 0.548s

OK (skipped=4)
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name='/dev/null' mode='w' encoding='UTF-8'>
```