Feature/issue 1052 runtests python

#### Summary:

Fixes a problem for windows machines when running too many tests.
```
runTests.py src/test/unit-distribution/univariate/continuous/skew_normal
```
used to fail.

#### Intended Effect:

The make command will get batches of tests of size `batchSize`. This breaks apart the size of the command ultimately submitted to Windows.

#### How to Verify:

On a Windows machine, type
` > runTests.py src/test/unit-distribution/univariate/continuous/skew_normal`

There are 164 generated tests. These get split into batches of 25 with a final batch of 14.

#### Side Effects:

No.

#### Documentation:

No user facing doc.

#### Reviewer Suggestions:

Anyone.