[SPARK-9464][SQL] Property checks for UTF8String

This PR is based on the original work by JoshRosen in #7780, which adds ScalaCheck property-based tests for UTF8String.

Author: Josh Rosen <joshrosen@databricks.com>
Author: Yijie Shen <henry.yijieshen@gmail.com>

Closes #7830 from yjshen/utf8-property-checks and squashes the following commits:

593da3a [Yijie Shen] resolve comments
c0800e6 [Yijie Shen] Finish all todos in suite
52f51a0 [Josh Rosen] Add some more failing tests
49ed0697 [Josh Rosen] Rename suite
9209c64 [Josh Rosen] UTF8String Property Checks.