[jdk11u-dev] RFR: 8301167: Update VerifySignedJar to actually exercise and test verification

[Amos]

- Backport [JDK-8301167](https://bugs.openjdk.org/browse/JDK-8301167)
- The patch applies clean, but the new code must be changed to Java 11 syntax

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Commit messages:
 - 8301167: Fix compile issue on Java 11 syntax
 - Backport 05ea083b0563ddacf3e38dc329ba00dc4bac9b29

Changes: https://git.openjdk.org/jdk11u-dev/pull/2185/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2185&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301167
  Stats: 111 lines in 2 files changed: 85 ins; 12 del; 14 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2185.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2185/head:pull/2185

PR: https://git.openjdk.org/jdk11u-dev/pull/2185