Here are patches to install a copy of xxmodule.c in the distutils tests directory (for 3.3, 3.2, and 2.7) and a copy in packaging tests for 3.3.  With them in place, test_build_ext/test_command_build_ext now executes when the tests are run from an installed Python where no source directory is available and each test case run copies the c file into its temp dir.
 
Currently, those test are being silently skipped in the installed case or, worse, picking up the wrong version of xxmodule.c depending on what sysconfig.get_config_vars('srcdir') returns (this happened to me when I reused a source directory name in some builds).   For 3.2, a patch is included to backport the Distutils part of the fix for issue12132.