Removed convertRomanNumeral as we are handling new conditions

I thought it made things a bit harder to understand, so I decided to
remove it first.
Return 0 if input has both supported and unsupported roman numerals

Our test before only handled if input as unsupported, and not a mix.
We should not be able to deduct I from V more than once
Show that we can handle all invalid multiple subtractions
Group related tests together
Removed test for single unsupported character as it is redundant
Name repeat tests explicitly
Name subtraction tests explicitly
Name invalid repeat tests explicitly
Name repeated too much tests explicitly
Name invalid subtraction tests explicitly
Put Multiple Subtraction before the specific condition