Provide a larger number of unit tests for better test coverage.

[gregtour]

The current testcases are: (as of rev. e4cbd55)

$ unittest test/testcases.txt
Running 20 tests...
1. simple-test [pass]
2. empty-test [pass]
3. print-ln-test [pass]
4. fibonacci-test-1 [pass]
5. fibonacci-test-2 [pass]
6. factorial-test [pass]
7. dictionary-array [pass]
8. arithmetic-1 [pass]
9. arithmetic-2 [pass]
10. hello-string [pass]
11. simple-prime [pass]
12. conditional-logic [pass]
13. halting-test [pass]
14. unicode-encoding [pass]
15. pi-digits [pass]
16. y-combinator [pass]
17. print-stars [pass]
18. lexical-scope [pass]
19. closure-test [pass]
20. mccarthy-91 [pass]
All passed!