RFR: 8375458: Check legal folder of JDK image for unwanted files [v2]

[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))

Matthias Baesken has updated the pull request incrementally with one additional commit since the last revision:

  GPL JDK image has different files in the legal subfolders

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Changes:
  - all: https://git.openjdk.org/jdk/pull/29272/files
  - new: https://git.openjdk.org/jdk/pull/29272/files/ef25393f..bf743de2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29272&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29272&range=00-01

  Stats: 24 lines in 1 file changed: 21 ins; 2 del; 1 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