tool/lib/test/unit: support TESTS='-- -ext-' again

There is a test directory named test/-ext-.  Because this directry
starts with a hyphen, we have to cheat test/unit in order for it to
run the tests underneath.  TESTS='-- -ext' worked for a long time.
Let's not break that maneuver.

tool/lib/test/unit.rb

@@ -70,7 +70,7 @@ module Test
         else
           seed = options[:seed] = srand % 100_000
           srand(seed)
-          orig_args << "--seed=#{seed}"
+          orig_args.unshift "--seed=#{seed}"
         end
 
         @help = "\n" + orig_args.map { |s|