rename ResultSet into ResultLister

I'm planning to rename Tescha::Pack into Tescha::Set. This commit is to
disambiguate with the new Set.
What's more, ResultLister might not contains a set of results actually
by the following changes I'm going to make.
rename Pack into Set

- 1 character shorter!
- really sounds like "a group of tests"
- I like "subset" better than "subpack"