RFR: 8282007: Assorted enhancements to jpackage testing framework

[Alexey Semenyuk]

8282007: Assorted enhancements to jpackage testing framework

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Commit messages:
 - Trailing whitespace removed
 - 8282007: Assorted enhancements to jpackage testing framework
 - macOS bugix
 - Don't use TKit.assertZZZ to validate test setup correctness. Throw exception instead.
 - Assorted fixes and enhancements to jpackage testing framework

Changes: https://git.openjdk.java.net/jdk/pull/7502/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7502&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282007
  Stats: 738 lines in 17 files changed: 413 ins; 180 del; 145 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7502.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7502/head:pull/7502

PR: https://git.openjdk.java.net/jdk/pull/7502