[jdk11u-dev] RFR: 8240903: Add test to check that jmod hashes are reproducible

[Christoph Langer]

This backports a test which should have been added with JDK-8240734 in first place.
I had to make a few modifications to make it work in 11u-dev, to be seen in the second commit.
I verified that the test fails without the changes of 8240734 and passes now with it.

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Commit messages:
 - Modifications
 - 8240903: Add test to check that jmod hashes are reproducible

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/1115/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=1115&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8240903
  Stats: 162 lines in 1 file changed: 162 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/1115.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/1115/head:pull/1115

PR: https://git.openjdk.java.net/jdk11u-dev/pull/1115