RFR: 8356562: SigningAppImageTwoStepsTest test fails

[Alexey Semenyuk]

Fix JPackageCommand.assertAppImageFile() method to make it pass the relative path to ".jpackag.xml" file to JPackageCommand.assertFileInAppImage().

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Commit messages:
 - Fix a bug in JPackageCommand.assertFileInAppImage() uncovered by running signing tests after JDK-8356219 fix. JDK-8356219 added a legit assert to JPackageCommand.assertFileInAppImage() and it failed.

Changes: https://git.openjdk.org/jdk/pull/25126/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25126&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356562
  Stats: 13 lines in 1 file changed: 6 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/25126.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25126/head:pull/25126

PR: https://git.openjdk.org/jdk/pull/25126