Adding a unique constraint to a column that already
has a foreign constraint, fails.

The problem was that the handler while copying the
FK metadata to a new table during the offline alter,
looks for an index with the column name that the FK
is based on and fails.

This patch fixes it by replacing the global index
lookup using column name with a lookup that takes in
the columns of the fk and returns a matching index
based on those columns. Also updated the testcases.