RFR: JDK-8246792: Mac signing tests failed (unsealed contents present in the bundle root)

[Alexey Semenyuk]

Please review fix [2] for jpackage bug [1].

Move `.jpackage.xml` file in `app` directory of app image and keep it in 
all OSX packages (dmg and pkg).
Resotore copying of `libjli.dylib` into `$RUNTIME/Contents/MacOS` 
directory in MacAppImageBuilder.java.
Update jtreg test helpers accordingly.

- Alexey

[1] https://bugs.openjdk.java.net/browse/JDK-8246792

[2] http://cr.openjdk.java.net/~asemenyuk/8246792/webrev.00