JDK-8009115: jtreg tests under jdk/test/javax/script should use nashorn as script engine 

Changes:

* Use "nashorn" engine name rather than "js" name to avoid possible
clash with other "js" engines such as rhino.

* load("nashorn:mozilla_compat.js") for tests that use importPackage

* Nashorn does not support accessing static fields from an object. Change to
use class name (javax.script.ScriptContext) to access static fields.

* Changed VersionTest.java to use Nashorn's language and engine versions.

* Expliciting calling removeAttribute on ScriptContext rather than depending
on null value for absence.

* Nashorn has no println. println->print change.

* Disabling GetInterfaceTest.java by jtreg @run ignore. This is a bug in nashorn