RFR: Add simple test for nullability warnings

[Maurizio Cimadamore]

This PR adds a simple test to validate the code in `Types` that generates the various nullability warnings.

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Commit messages:
 - Add null warning test

Changes: https://git.openjdk.org/valhalla/pull/1220/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1220&range=00
  Stats: 63 lines in 2 files changed: 63 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1220.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1220/head:pull/1220

PR: https://git.openjdk.org/valhalla/pull/1220