Fix run-one-test so that it runs elf tests

The `test' make target passes a trailing slash in the subdir argument.  This
does not play well with elf/rtld-Rules which looks for `elf' without any
trailing slash, and therefore doesn't find a match when running an elf test
individually.  This commit removes the trailing slash from the invocation.

Reviewed-by: DJ Delorie <dj@redhat.com>