[SPARK-21837][SQL][TESTS] UserDefinedTypeSuite Local UDTs not actually testing what it intends

## What changes were proposed in this pull request?

Adjust Local UDTs test to assert about results, and fix index of vector column. See JIRA for details.

## How was this patch tested?

Existing tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #19053 from srowen/SPARK-21837.