duplicity: enable tests

* Install required tool dependencies ('tar', 'rdiff', etc.)
* Run installed duplicity instead of source tree duplicity
* Fix shebang of scripts in testing/overrides/bin/
* Disable some unhelpful developer-only tests
* Disable a test broken on Linux (root cause unknown)
* Work around low file descriptor soft limit for Darwin builders

pkgs/tools/backup/duplicity/use-installed-scripts-in-test.patch

@@ -0,0 +1,13 @@
+--- a/setup.py
++++ b/setup.py
+@@ -92,10 +92,6 @@ class TestCommand(test):
+                 except Exception:
+                     pass
+
+-        os.environ['PATH'] = "%s:%s" % (
+-            os.path.abspath(build_scripts_cmd.build_dir),
+-            os.environ.get('PATH'))
+-
+         test.run(self)
+
+     def run_tests(self):