[jdk11u-dev] RFR: 8231427: Warning cleanup in tests of java.io.Serializable

[Andrew Lu]

I backport this for parity with 11.0.25-oracle.

-------------

Commit messages:
 - Backport 942402bab8a632f9e8a9b0c56201e2ab0571e6e8

Changes: https://git.openjdk.org/jdk11u-dev/pull/2825/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2825&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8231427
  Stats: 630 lines in 114 files changed: 317 ins; 5 del; 308 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2825.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2825/head:pull/2825

PR: https://git.openjdk.org/jdk11u-dev/pull/2825