Do not test java.util.TreeMap's serialization in the Serialize test

In the Android class path, TreeMap is implemented differently and as a
consequence its serialization is incompatible with OpenJDK's. So let's
test a private static class' serialization instead, to make sure that
the wire protocol defined by the Java Language Specification is
implemented.

This addresses issue #123 reported by Joel Dice.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>