[SPARK-1455] [SPARK-3534] [Build] When possible, run SQL tests only.

If the only files changed are related to SQL, then only run the SQL tests.

This patch includes some cosmetic/maintainability refactoring. I would be more than happy to undo some of these changes if they are inappropriate.

We can accept this patch mostly as-is and address the immediate need documented in [SPARK-3534](https://issues.apache.org/jira/browse/SPARK-3534), or we can keep it open until a satisfactory solution along the lines [discussed here](https://issues.apache.org/jira/browse/SPARK-1455?focusedCommentId=14136424&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14136424) is reached.

Note: I had to hack this patch up to test it locally, so what I'm submitting here and what I tested are technically different.

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

Closes #2420 from nchammas/selective-testing and squashes the following commits:

db3fa2d [Nicholas Chammas] diff against master!
f9e23f6 [Nicholas Chammas] when possible, run SQL tests only