Moved tests related to interpolation.

The tests affected test basic interpolation functionality. Therefore, they were
moved from TestPropertyConverter to TestAbstractConfigurationBasicFeatures.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1515442 13f79535-47bb-0310-9956-ffa450edef68
Inlined the interpolate() helper method from PropertyConverter.

This was the only place this method was used. The plan is to remove
interpolate() from PropertyConverter, so that this class can focus only on
conversions.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1515443 13f79535-47bb-0310-9956-ffa450edef68