return-value of mrb_run is invalid in top-level-scope

The return-value of mrb_run in top-level-scope
should be the evaluated value at last.
add String#partition and String#rpartition.
More singleton tests
When superclass isn't a class tests
add some tests for [], delete_at, index, and rindex
Merge pull request #1629 from akuroda/add_array_test

add tests for Array methods
Merge pull request #1626 from carsonmcdonald/moreclasstests

More tests for Class behavior
Merge pull request #1625 from iij/pr-string-partition

add String#partition and String#rpartition.
Merge pull request #1624 from cubicdaiya/issues/invalid_return_in_top_level

return-value of mrb_run is invalid in top-level-scope