RFR: 8231591: [TESTBUG] Create additional two phase jpackage tests

[Alexander Matveev]

https://bugs.openjdk.java.net/browse/JDK-8231591

- Added MultiLauncherTwoPhaseTest which uses predefine app image with multiple launcher and tests to make sure installer
  will create shortcuts for all launchers.
- Fixed Linux DesktopIntegration to create shortcuts for additional launcher if we using pre-define app image.

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Commit messages:
 - 8231591: [TESTBUG] Create additional two phase jpackage tests

Changes: https://git.openjdk.java.net/jdk/pull/263/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=263&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8231591
  Stats: 173 lines in 5 files changed: 164 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/263.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/263/head:pull/263

PR: https://git.openjdk.java.net/jdk/pull/263