test that readonly directory give error

ie. when cmdtest fails to remove the "work"
directory between test methods. Previously
cmdtest relied on FileUtils.rm_rf, but it could
fail silently, leading to weird consequences.

t/CMDTEST_readonly.rb

@@ -0,0 +1,33 @@
+
+require "selftest_utils"
+
+class CMDTEST_readonly < Cmdtest::Testcase
+
+  include SelftestUtils
+
+  def teardown
+    File.chmod(0755, 'tmp-cmdtest-2/top/work/a_subdir')
+  end
+
+  def test_readonly
+    create_CMDTEST_foo [
+      "cmd 'true.rb' do",
+      "end",
+      "Dir.mkdir('a_subdir')",
+      "File.open('a_subdir/file1', 'w') {|f| f.puts 123}",
+      "File.chmod(0, 'a_subdir')",
+    ]
+
+    cmd_cmdtest do
+      stdout_equal [
+        "### true.rb",
+      ]
+    end
+
+    cmd_cmdtest do
+      stderr_equal /Directory not empty/
+      exit_nonzero
+    end
+  end
+
+end