StrSearcher: Add tests for rfind(&str)
StrSearcher: Add tests for rfind(&str)

Add tests for .rfind(&str), using the reverse searcher case for
substring search.

aidancully/rust,GBGamer/rust,dwillmer/rust,dwillmer/rust,dwillmer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,dwillmer/rust,graydon/rust,GBGamer/rust,dwillmer/rust,dwillmer/rust,graydon/rust,dwillmer/rust,GBGamer/rust,GBGamer/rust,dwillmer/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust