Python: move test modules and data to python3-flightgear/flightgear/meta/tests/

- Move test_catalog.py, test_sgprops.py and catalog/testData/ to
  python3-flightgear/flightgear/meta/tests/.
- Copy catalog/fgaddon-catalog/ to the same place (it is needed by
  test_catalog.py).
- Adapt test_catalog.py and test_sgprops.py accordingly.
- Remove the executable bits from test_catalog.py and test_sgprops.py:
  this is because test_catalog.py can't be run directly anymore; well,
  it can but 3 tests would fail in this setup. No time to investigate
  why (sorry), but this commit adds a README.txt in
  python3-flightgear/flightgear/meta/tests/README.txt that explains how
  to run the tests in a way that works (basically, run
  'python3 -m unittest' from the python3-flightgear directory).

For a bit more context, see
<https://sourceforge.net/p/flightgear/mailman/message/37042247/>.

python3-flightgear/flightgear/meta/tests/test_sgprops.py

@@ -0,0 +1,70 @@
+#! /usr/bin/env python3
+
+import os
+import unittest
+
+from flightgear.meta import sgprops
+
+
+baseDir = os.path.dirname(__file__)
+
+def testData(*args):
+    return os.path.join(baseDir, "testData", *args)
+
+
+class SGProps(unittest.TestCase):
+
+    def test_parse(self):
+        parsed = sgprops.readProps(testData("props1.xml"))
+
+        self.assertEqual(parsed.getValue("value"), 42)
+        self.assertEqual(type(parsed.getValue("value")), int)
+
+        valNode = parsed.getChild("value")
+        self.assertEqual(valNode.parent, parsed)
+        self.assertEqual(valNode.name, "value")
+
+        self.assertEqual(valNode.value, 42)
+        self.assertEqual(type(valNode.value), int)
+
+        with self.assertRaises(IndexError):
+            missingNode = parsed.getChild("missing")
+
+        things = parsed.getChildren("thing")
+        self.assertEqual(len(things), 3)
+
+        self.assertEqual(things[0], parsed.getChild("thing", 0));
+        self.assertEqual(things[1], parsed.getChild("thing", 1));
+        self.assertEqual(things[2], parsed.getChild("thing", 2));
+
+        self.assertEqual(things[0].getValue("value"), "apple");
+        self.assertEqual(things[1].getValue("value"), "lemon");
+        self.assertEqual(things[2].getValue("value"), "pear");
+
+    def test_create(self):
+        pass
+
+
+    def test_invalidIndex(self):
+        with self.assertRaises(IndexError):
+            parsed = sgprops.readProps(testData("bad-index.xml"))
+
+    def test_include(self):
+        parsed = sgprops.readProps(testData("props2.xml"))
+
+        # test that value in main file over-rides the one in the include
+        self.assertEqual(parsed.getValue("value"), 33)
+
+        # but these come from the included file
+        self.assertEqual(parsed.getValue("value[1]"), 43)
+        self.assertEqual(parsed.getValue("value[2]"), 44)
+
+        subNode = parsed.getChild("sub")
+        widgets = subNode.getChildren("widget")
+        self.assertEqual(len(widgets), 4)
+
+        self.assertEqual(widgets[2].value, 44)
+        self.assertEqual(widgets[3].value, 99)
+
+if __name__ == '__main__':
+    unittest.main()