Creating a HashMap directly from JNI is possible but very tedious
(see: http://java.sun.com/docs/books/jni/html/fldmeth.html#26254)

Instead we use the existing code to return hashes from JNI as plain
String[], then add some code in the Java wrapper to convert these to
HashMap<String,String>.

There's no point returning the specific HashMap type here.
Return the generic interface type instead.

Note that no users are actually calling these functions yet,
since at present they always fail.

In functions that don't have javadoc, the function prototype wasn't
being indented correctly.

If a test.img file was left over from a previous run, then it
would cause the subsequent test to fail.  Therefore remove any
old test.img file.

It turns out that Java assertions are disabled by default.  You have
to add the 'java -ea' flag to the JVM.  Who knew ..?

Because of this oversight, the tests weren't actually performing the
assertions that we wanted (although in fact none of the assertions
were failing).

This change enables assertions when running the tests.

This updates commit 7eb012f3710bb554d5fc2c4229036901b0b5ad90.