[SPARK-8446] [SQL] Add helper functions for testing SparkPlan physical operators

This patch introduces `SparkPlanTest`, a base class for unit tests of SparkPlan physical operators.  This is analogous to Spark SQL's existing `QueryTest`, which does something similar for end-to-end tests with actual queries.

These helper methods provide nicer error output when tests fail and help developers to avoid writing lots of boilerplate in order to execute manually constructed physical plans.

Author: Josh Rosen <joshrosen@databricks.com>
Author: Josh Rosen <rosenville@gmail.com>
Author: Michael Armbrust <michael@databricks.com>

Closes #6885 from JoshRosen/spark-plan-test and squashes the following commits:

f8ce275 [Josh Rosen] Fix some IntelliJ inspections and delete some dead code
84214be [Josh Rosen] Add an extra column which isn't part of the sort
ae1896b [Josh Rosen] Provide implicits automatically
a80f9b0 [Josh Rosen] Merge pull request #4 from marmbrus/pr/6885
d9ab1e4 [Michael Armbrust] Add simple resolver
c60a44d [Josh Rosen] Manually bind references
996332a [Josh Rosen] Add types so that tests compile
a46144a [Josh Rosen] WIP