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/Aircraft/f16/f16b-set.xml

@@ -0,0 +1,29 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<PropertyList include="f16-common.xml">
+  <sim>
+    <name>f16b</name>
+    <description>F16-B</description>
+    <long-description>The F16-B is an upgraded version of the F16A.</long-description>
+    <variant-of>f16a</variant-of>
+
+    <localized>
+      <de>
+        <long-description>Describe the F16-B in German</long-description>
+      </de>
+      
+      <fr>
+              <long-description>Describe the F16-B in French</long-description>
+      </fr>
+    </localized>
+
+    <authors n="0">
+      <author n="0">
+        <name>James T Kirk</name>
+        <email>shatner@enterprise.com</email>
+        <nick>starlover</nick>
+        <description>Everything</description>
+      </author>
+    </authors>
+  </sim>
+
+</PropertyList>