ConstantSchemaAbstractDataBrokerTest, faster than AbstractDataBrokerTest

ConstantSchemaAbstractDataBrokerTest, faster than AbstractDataBrokerTest

In something like AclServiceTest, of which I'm going to be writing a lot
more shortly, which runs e.g. 7 @Test methods under a fresh DataBroker,
there is no need to re-re-re-re do the bloody slow creation of
SchemaContext every time (because its content is based on the classpath,
wihch is fixed for a given single test).

This optimization approx. halfs the time e.g. for AclServiceTest, from
ca. 30s to ca. 15s.

Implemented on the train & ferry traveling back from EclipseCon ;)

Change-Id: Ic47f8abf9833bafcce13655b46cbce3e02aed050
Signed-off-by: Michael Vorburger <vorburger@redhat.com>