workload/schemachanger: expect competing drop error in operation generation

The type-specific table dropped error (#169991)
needs to be handled in the operation generator.

Epic: none
Fixes: #169638

Release note: None

Author: bghal

pkg/workload/schemachange/operation_generator.go

@@ -217,6 +217,12 @@ func (og *operationGenerator) randOp(
 			if errors.Is(err, ErrSchemaChangesDisallowedDueToPkSwap) {
 				continue
 			}
+			// A concurrent DROP can remove a descriptor while we are generating
+			// an operation against it.
+			if pgErr := new(pgconn.PgError); errors.As(err, &pgErr) &&
+				pgErr.Code == pgcode.UndefinedTable.String() {
+				continue
+			}
 			return nil, errors.Wrapf(err, "failed generating operation: %s", op)
 		}