tests: Check that generated sources can be used in csharp targets

test cases/csharp/2 library/meson.build

@@ -1,5 +1,15 @@
 project('C# library', 'cs')
 
-l = shared_library('helper', 'helper.cs', install : true)
+python3 = import('python3').find_python()
+generated_sources = custom_target('gen_sources',
+    input: 'helper.cs',
+    output: 'helper.cs',
+    command: [python3, '-c',
+      'import shutil, sys; shutil.copyfile(sys.argv[1], sys.argv[2])',
+      '@INPUT@', '@OUTPUT@']
+)
+
+l = shared_library('helper', generated_sources, install : true)
+
 e = executable('prog', 'prog.cs', link_with : l, install : true)
 test('libtest', e)