[jdk21u-dev] RFR: 8343875: Minor improvements of jpackage test library

[Goetz Lindenmaier]

I backport this to keep the jpackage testing infra up to date.  Simplifies later test backports. 

Two files had conflicts:

test/jdk/tools/jpackage/helpers-test/jdk/jpackage/test/DirectoryContentVerifierTest.java
Was added by 8325089: "jpackage utility creates an "infinite", undeleteable directory tree",
too big to backport.  Omit patch.

test/jdk/tools/jpackage/helpers/jdk/jpackage/test/PackageTest.java
Resolved one chunk due to modifier order.  Resolved to the order used in 25 in this place.

And I needed some adaption, again as 8325089: "jpackage utility creates an "infinite", undeleteable directory tree"
is missing.  I added one chunk from 8325089 to TKitTest.java and I added assertAssert() to TKit.java.

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Commit messages:
 - Add on from JDK-8325089
 - backport 95a00f8a188048952871a10dc428566b18b91cb8

Changes: https://git.openjdk.org/jdk21u-dev/pull/2315/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2315&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343875
  Stats: 546 lines in 10 files changed: 491 ins; 21 del; 34 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2315.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2315/head:pull/2315

PR: https://git.openjdk.org/jdk21u-dev/pull/2315