build: Switch order of subdirectories

This makes demos be compiled/linked before tests. And that means that
while hacking, I can already run widget-factory when the tests are
still linking.

meson.build

@@ -593,14 +593,14 @@ subdir('gdk')
 subdir('gsk')
 subdir('gtk')
 subdir('modules')
-if get_option('build-tests')
-  subdir('tests')
-  subdir('testsuite')
-endif
 if get_option('demos')
   subdir('demos')
   subdir('examples')
 endif
+if get_option('build-tests')
+  subdir('tests')
+  subdir('testsuite')
+endif
 
 # config.h
 configure_file(input: 'config.h.meson',