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/testData/props1.xml

@@ -0,0 +1,25 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<PropertyList>
+  <value type="int">42</value>
+
+  <thing>
+      <value>apple</value>
+  </thing>
+
+  <thing>
+    <value>lemon</value>
+  </thing>
+
+  <thing>
+    <value>pear</value>
+  </thing>
+
+  <!-- ensure explicit indexing works -->
+  <sub>
+      <value n="1">a</value>
+      <value n="3">b</value>
+      <value n="99">c</value>
+      <value>d</value> <!-- should be assigned n=100 -->
+  </sub>
+
+</PropertyList>