Do not rmdir test directories

The old behavior is not useful when you are in the test output
directory.
diff --git a/runtest.rb b/runtest.rb
index def12e9..3fb73a5 100755
--- a/runtest.rb
+++ b/runtest.rb
@@ -21,8 +21,11 @@
 
   name = mk[/([^\/]+)\.mk$/, 1]
   dir = "out/#{name}"
-  FileUtils.rm_rf(dir)
+
   FileUtils.mkdir_p(dir)
+  Dir.glob("#{dir}/*").each do |fname|
+    FileUtils.rm_rf(fname)
+  end
 
   Dir.chdir(dir) do
     File.open("Makefile", 'w') do |ofile|