Added a function to construct a illegalMessage for negative numbers and fixed the test itself
Fixed the function that constructs illegalMessage, forgot to add the commas and trim the last one off. Now the implementation works fine for negative numbers
Forgot to mention that I implemented the check for negative numbers in the function sum(String[] numbers), comments added there
Added the test numbersBiggerThan1000
Fixed the quotes in numbersBiggerThan1000 test  and implemented the calculations in the function sum