rethink approach...
Do most work in a process method, see initial comment in FractionCalculator
for details. Implement testing in FractionCalculatorTest as it is clearer.
Change static variables like MULTIPLY to char rather than string as
this be clearer.

Test output for this skeleton:

$ java -ea FractionCalculatorTest
FractionTestCalculator tests verbose output:
debug call to process with inputString ="quit"