In BaseContextSenstiveTest, make dropNotNullConstraint method protected

In 2.3 we needed to add a slight hack to remove the not-null constraint from Person when testing:

https://github.com/openmrs/openmrs-core/blob/master/api/src/test/java/org/openmrs/test/BaseContextSensitiveTest.java#L587

We should make the class to remove not null constraints public so that extending classes can reference it.