Test files in the same directory share the same %T. %T is easy to
misuse and cause race conditions (when running concurrently) so it has
been deprecated since D48842 (see docs/CommandGuide/lit.rst).
While here, add rm -rf %t.dir so that tests cannot depend on old files lying around.
Aside: Shouldn't this be REQUIRES: x86, system-linux (or whatever the equivalent is), rather than REQUIRES: x86, shell?