[lworld] RFR: 8328461: [lworld] Reimplement FieldAccessModify test for JEP401

[Alex Menkov]

- updated the test for field access/modify (use value classes);
- enabled testing of FieldModify events (the functionality was fixed);
- renamed "Valhalla" to "valhalla" for consistency with other tests.

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Commit messages:
 - Merge branch 'FieldAccessModify' of github.com:alexmenkov/valhalla into FieldAccessModify
 - update problemlist
 - rename Valhalla dir to valhalla
 - update FieldAccessModify test
 - update problemlist
 - rename Valhalla dir to valhalla
 - update FieldAccessModify test

Changes: https://git.openjdk.org/valhalla/pull/1241/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1241&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328461
  Stats: 445 lines in 5 files changed: 219 ins; 225 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/1241.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1241/head:pull/1241

PR: https://git.openjdk.org/valhalla/pull/1241