RFR: 8231591: [TESTBUG] Create additional two phase jpackage tests [v3]

[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.

Alexander Matveev has updated the pull request incrementally with one additional commit since the last revision:

  8231591: [TESTBUG] Create additional two phase jpackage tests (revision 2)

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/263/files
  - new: https://git.openjdk.java.net/jdk/pull/263/files/5303fcc0..650cf21e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=263&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=263&range=01-02

  Stats: 20 lines in 1 file changed: 10 ins; 9 del; 1 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