RFR: 8264512: jdk/test/jdk/java/util/prefs/ExportNode.java relies on default platform encoding
This test emits to a `java.io.ByteArrayOutputStream` the contents of a `java.utils.prefs.Preferences` node. The `UTF-8` character encoding is used . The `ByteArrayOutputStream` is then converted to a `String` using `toString()` which uses the platform's default character set . The resulting `String` is then checked for the node names that it should and should not contain.
This change proposes to use `ByteArrayOutputStream.toString(String)` to obtain the string  to maintain consistency of the encoding. It also adds throwing a `RuntimeException` if the node string is not as expected. It is unclear why this test was not already throwing such an exception.