Fix extern_string_test*.chpl and remove .future, .bad, and .prediff

[ok'ed by Michael]

With r23558 (explicit separation between c_string and Chapel string),
it is possible to properly write these test programs that return and
allocate strings.

Removed the .future and .bad files, as well as the .prediff which was
to quiet non-deterministic output of the future.



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23790 3a8e244f-b0f2-452b-bcba-4c88e055c3ca