Add a test that demonstrates the problem.
Cannot remove group loop initial table if it's the only one, even if columns unused. Its rows determine cardinality.

Fixes #1315
merge MMcM: Cannot remove group loop target table if it's the only table in group

Merge pull request #1316 from MMcM/sql-layer/sql-group-loop-single-unreferenced-table
https://github.com/FoundationDB/sql-layer/pull/1316