[SQL] Fixes caching related JoinSuite failure

PR #2860 refines in-memory table statistics and enables broader broadcasted hash join optimization for in-memory tables. This makes `JoinSuite` fail when some test suite caches test table `testData` and gets executed before `JoinSuite`. Because expected `ShuffledHashJoin`s are optimized to `BroadcastedHashJoin` according to collected in-memory table statistics.

This PR fixes this issue by clearing the cache before testing join operator selection. A separate test case is also added to test broadcasted hash join operator selection.

Author: Cheng Lian <lian@databricks.com>

Closes #2960 from liancheng/fix-join-suite and squashes the following commits:

715b2de [Cheng Lian] Fixes caching related JoinSuite failure