test: use Test::Unit::TestCase.ractor

Instead of using assert_separately. If we use .ractor, we can use the
same process for tests that use Ractor. It's easy to debug.

test/pathname/test_ractor.rb

@@ -3,20 +3,14 @@
 require "pathname"
 
 class TestPathnameRactor < Test::Unit::TestCase
-  def setup
-    omit unless defined? Ractor
-  end
-
-  def test_ractor_shareable
-    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
-    begin;
-      $VERBOSE = nil
-      require "pathname"
+  if respond_to?(:ractor)
+    ractor
+    def test_shareable
       r = Ractor.new Pathname("a") do |x|
         x.join(Pathname("b"), Pathname("c"))
       end
       assert_equal(Pathname("a/b/c"), r.take)
-    end;
+    end
   end
 end