DefaultCalibratedRealIntervalTest: fix up test1

This change narrows the scope of the test to focus on
DefaultCalibratedRealInterval only, rather than including tests for
LinearAxis (which already has its own unit test class: LinearAxisTest).

It also tests for null axes, which should be the case initially after
using the DefaultCalibratedRealInterval(double[]) constructor.
DefaultCalibratedRealIntervalTest: clean up style