tests: split/l-chunk: avoid a portability issue

* tests/split/l-chunk: Don't use the `test "$var"`
idiom to test that var is set to something as
that's not supported by all shells.
The new style matches the usage is the rest of
the test in any case.
Reported by Bruno Haible on AIX 6.1 and 7.1

tests/split/l-chunk

@@ -108,7 +108,7 @@ for ELIDE_EMPTY in '' '-e'; do
         echo "$lines"
       fi
     done
-    test "$ELIDE_EMPTY" && EXP=exp.elide_empty || EXP=exp
+    test -z "$ELIDE_EMPTY" && EXP=exp || EXP=exp.elide_empty
     compare out $EXP || fail=1
   done
 done