[jdk11u-dev] RFR: 8209935: Test to cover CodeSource.getCodeSigners()

[Goetz Lindenmaier]

I backport this for parity with 11.0.19-oracle.

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Commit messages:
 - Backport 0cb0ecf4433f1054ba2f0fbdabee01323893e0fe

Changes: https://git.openjdk.org/jdk11u-dev/pull/1697/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1697&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8209935
  Stats: 157 lines in 3 files changed: 149 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1697.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1697/head:pull/1697

PR: https://git.openjdk.org/jdk11u-dev/pull/1697