test: skip test rather than fail with Solaris 10 sed

* tests/rm/r-root.sh: This test was seen to ERROR due to
sed input not having a terminating '\n'.  Therefore just skip
with diagnostics upon failure to adjust the error output with sed.
Reported by Assaf Gordon.

tests/rm/r-root.sh

@@ -142,7 +142,7 @@ exercise_rm_r_root ()
 
   ret=$?
 
-  clean_rm_err_ < err.t > err || framework_failure_
+  clean_rm_err_ < err.t > err || ret=$?
 
   return $ret
 }