Randomizer/Source: fix a wrong filename
Randomizer/Source: add a test case for SourceInterface

A little redundant code is also moved into the test case for the interface.
Randomizer: make a local variable name more readable
Randomizer: add getMin(), getMax() interfaces
Randomizer: generate() accepts null as the interface
Randomizer: generate() throws DomainException as the interface
Randomizer: add a test case for RandomizerInterface

Tests for the interface is moved into the test case for the interface.