Compile wxConfig unit tests as part of non-GUI test too

Almost none of the tests require GUI, so make it possible to run them
without it, but still include the GUI tests when using GUI.

tests/test.bkl

@@ -34,6 +34,7 @@
             arrays/arrays.cpp
             base64/base64.cpp
             cmdline/cmdlinetest.cpp
+            config/config.cpp
             config/fileconf.cpp
             config/regconf.cpp
             datetime/datetimetest.cpp
@@ -187,6 +188,9 @@
             graphics/graphmatrix.cpp
             graphics/graphpath.cpp
             graphics/imagelist.cpp
+            <!--
+                Duplicate this file here to compile a GUI test in it too.
+             -->
             config/config.cpp
             controls/auitest.cpp
             controls/bitmapcomboboxtest.cpp