Fix for NMS-6278 Part III - Fixing failing tests.

The method thresholdEntity.getRequiredDatasources() returns all the
required data sources (from the expression and the filters).

The method thresholdEntity.getThresholdConfig().getRequiredDatasources()
returns only the datasources used on the expression, which is what the
deprecated SnmpThresholder and the JMXThresholder expects to use.