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/f16-common.xml

@@ -0,0 +1,24 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<PropertyList>
+  <sim include="settings-common.xml">
+    <authors>
+      <author n="0">
+        <name>Wilbur Wright</name>
+        <email>ww@wright.com</email>
+        <nick>wilburw</nick>
+        <description>Model, FDM and cockpit</description>
+      </author>
+      <author n="1">
+        <name>Orville Wright</name>
+        <description>Testing and systems</description>
+      </author>
+    </authors>
+    <tags>
+      <tag>fighter</tag>
+      <tag>1980s</tag>
+      <tag>glass-cockpit</tag>
+    </tags>
+    <minimum-fg-version>2017.4</minimum-fg-version>
+  </sim>
+
+</PropertyList>