RFR: 8375458: Check legal folder of JDK image for unwanted files

[Matthias Baesken]

We currently expect certain files in the legal folder of the jdk image.
This should be checked by a test.
(recently a check for the bin and lib folders has been added by [JDK-8374471](https://bugs.openjdk.org/browse/JDK-8374471))

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Commit messages:
 - JDK-8375458

Changes: https://git.openjdk.org/jdk/pull/29272/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29272&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375458
  Stats: 42 lines in 1 file changed: 42 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29272.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29272/head:pull/29272

PR: https://git.openjdk.org/jdk/pull/29272