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/fgaddon-catalog/template.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--Template catalog - copy and modify for your site as required-->
+<PropertyList>
+  <template>
+    <version n="0">3.4.*</version>
+    <version n="1">3.5.*</version>
+    <version n="2">3.6.*</version>
+    <version n="3">3.7.*</version>
+    <version n="5">2016.*.*</version>
+    <version n="6">2017.*.*</version>
+    <version n="7">2018.*.*</version>
+    <id>org.flightgear.fgaddon</id>
+    <license>GPL</license>
+    <url>http://mirrors.ibiblio.org/flightgear/ftp/Aircraft/catalog.xml</url>
+    <name>FlightGear aircraft distribution from fgaddon</name>
+    <description>This hangar provides aircraft officially supported and maintained by the FlightGear project, under a free-software license.</description>
+    <de>
+      <description>Auf Deutsch</description>
+    </de>
+    <fr>
+      <description>En Francais</description>
+    </fr>
+  </template>
+</PropertyList>