Testing arguments to `sbt` need to be passed as an array, not a single, long string.

Fixes a bug introduced in #2420.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #2437 from nchammas/selective-testing and squashes the following commits:

a9f9c1c [Nicholas Chammas] fix printing of sbt test arguments
cf57cbf [Nicholas Chammas] fix sbt test arguments
e33b978 [Nicholas Chammas] Merge pull request #2 from apache/master
0b47ca4 [Nicholas Chammas] Merge branch 'master' of github.com:nchammas/spark
8051486 [Nicholas Chammas] Merge pull request #1 from apache/master
03180a4 [Nicholas Chammas] Merge branch 'master' of github.com:nchammas/spark
d4c5f43 [Nicholas Chammas] Merge pull request #6 from apache/master