RFR: 8256202: Some tweaks for jarsigner tests PosixPermissionsTest and SymLinkTest

[Christoph Langer]

Working on 11u backports of JDK-8218021 and JDK-8250968, I found some minor points for improvement in tests
test/jdk/sun/security/tools/jarsigner/PosixPermissionsTest.java and
test/jdk/sun/security/tools/jarsigner/SymLinkTest.java

The details

PosixPermissionsTest:
- it can run on any system, no matter if the default filesystem supports Posix or not since Posix support is only required for the zipfs which is always true.
- improve some comments for the test flow

SymLinkTest:
- make output of createByteArray prettier
- improve inline comments to ease understanding
- add an option to main for generating ZIPBYTES
- use "Files.write(Path.of(ZIPFILENAME), ZIPBYTES)" for creating the test zipfile

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Commit messages:
 - 8256202: Some tweaks for jarsigner tests PosixPermissionsTest and SymLinkTest

Changes: https://git.openjdk.java.net/jdk/pull/1166/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1166&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256202
  Stats: 91 lines in 2 files changed: 34 ins; 7 del; 50 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1166.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1166/head:pull/1166

PR: https://git.openjdk.java.net/jdk/pull/1166