RFR: 8268613: jar --validate should check inital entries of a JAR file [v2]

[Christian Stein]

> Please review this change to include a validation check for expecting an optional JAR manifest entry being the first or second entry in JAR file. As the second entry, it must be only preceeded by an entry for the `META-INF/` directory.

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Split test steps

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28154/files
  - new: https://git.openjdk.org/jdk/pull/28154/files/2af59381..28068ea6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28154&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28154&range=00-01

  Stats: 12 lines in 1 file changed: 6 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28154.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28154/head:pull/28154

PR: https://git.openjdk.org/jdk/pull/28154