[flang] Tests for DO loop semantics

These are tests for checks that are already implemented, but for which we
did not have tests.

Original-commit: flang-compiler/f18@68f5acf7b1202162150162e1974fe43341a33eed
Reviewed-on: https://github.com/flang-compiler/f18/pull/637
Tree-same-pre-rewrite: false