This should finally fix kv.test and dotty.test on the LIP6 buildfarm.

* src/tgbatest/kv.test, iface/nips/nipstest/dotty.test: Don't rely
on the ${DOT-...} syntax, because DOT is always set and might be
set to the empty value.

iface/nips/nipstest/dotty.test

@@ -26,6 +26,6 @@ set -e
 BYTECODE="never.pr.b regbit.b"
 for bytecode in $BYTECODE; do
     run 0 "$top_builddir/dottynips" "$srcdir/$bytecode" > dotty || exit 1
-    "${DOT-true}" -Tps dotty > /dev/null || exit 1
+    test -z "$DOT" || "$DOT" -Tps dotty > /dev/null || exit 1
     rm -f dotty
 done