Test: adding the rest two digits convert to one word cases, 40 to 90
TPP: constant -> constants (adding more pairs to WORDS map)
Test: 100
TPP: constant -> constants
And, small refactoring to remove the duplication of word "one"
Test: 101, passed directly without code change
Some refactoring for the test names
Test:121, three digits convert to one number string with two words and another two words
TPP: statement -> recursion