Tweak the test-alter-identity test to use DropTable command rather than
a statement.
Remove the drop tables.
merge tjoneslo: Tweak the test-alter-identity test to use DropTable command

Merge pull request #1390 from tjoneslo/sql-layer/fts-tweak-alter-identity
https://github.com/FoundationDB/sql-layer/pull/1390