DRILL-5518: Test framework enhancements
* Create a SubOperatorTest base class to do routine setup and shutdown.
* Additional methods to simplify creating complex schemas with field
widths.
* Define a test workspace with plugin-specific options (as for the CSV
storage plugin)
* When verifying row sets, add methods to verify and release just the
"actual" batch in addition to the existing method for verify and free
both the actual and expected batches.
* Allow reading of row set values as object for generic comparisons.
* "Column builder" within schema builder to simplify building a single
MatrializedField for tests.
* Misc. code cleanup.
closes #851
