Add developer docs about testing

Documentation/development/testing.md

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+# Testing KiCad #
+
+# Unit tests #
+
+KiCad has a limited number of unit tests, which can be used to
+check that certain functionality works.
+
+## Python modules ##
+
+The Pcbnew Python modules have some test programs in the `qa` directory.
+You must have the `KICAD_SCRIPTING_MODULES` option on in CMake to
+build the modules and enable this target.
+
+The main test script is `qa/test.py` and the test units are in
+`qa/testcases`. All the test units can by run using `make qa`, which
+runs `test.py`.
+
+You can also run an individual case manually, by making sure the
+modules are built, adding them to `PYTHONPATH` and running the test
+from the source tree:
+
+    make pcbnew_python_module
+    export PYTHONPATH=/path/to/kicad/build/pcbnew
+    cd /path/to/kicad/source/qa
+    python2 testcase/test_001_pcb_load.py
+
+### Diagnosing segfaults ###
+
+Although the module is Python, it links against a C++ library
+(the same one used by KiCad Pcbnew), so it can segfault if the library
+has a defect.
+
+You can run the tests in GDB to trace this:
+
+    $ gdb
+
+    (gdb) file python2
+    (gdb) run testcases/test_001_pcb_load.py
+
+If the test segfaults, you will get a familiar backtrace, just like
+if you were running pcbnew under GDB.