RFR: 8303959: tools/jpackage/share/RuntimePackageTest.java fails with java.lang.AssertionError missing files

[Alexey Semenyuk]

Don't use JDK image from `$JAVA_HOME` as a value of `--runtime-image` jpackage cli option. Use jlink to create a runtime in test work dir if the default runtime is not specified and pass the location of jlink output.

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Commit messages:
 - 8303959: tools/jpackage/share/RuntimePackageTest.java fails with java.lang.AssertionError missing files

Changes: https://git.openjdk.org/jdk/pull/15971/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15971&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303959
  Stats: 22 lines in 1 file changed: 18 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15971.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15971/head:pull/15971

PR: https://git.openjdk.org/jdk/pull/15971