[jdk11u-dev] RFR: 8300259: Add test coverage for processing of pending block files in signed JARs

[Amos]

- Backport of [JDK-8300259](https://bugs.openjdk.org/browse/JDK-8300259)
- Test succeeded in local dev box

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Commit messages:
 - Backport c129ce4660e6c9b5365c8bf89fb916e2a7c28e98

Changes: https://git.openjdk.org/jdk11u-dev/pull/2181/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2181&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300259
  Stats: 173 lines in 1 file changed: 173 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2181.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2181/head:pull/2181

PR: https://git.openjdk.org/jdk11u-dev/pull/2181