Issue #12141: Install copies of template C module file so that

test_build_ext of test_distutils and test_command_build_ext of
test_packaging are no longer silently skipped when
run outside of a build directory.

tests/test_build_ext.py

@@ -22,6 +22,11 @@ from test.support import run_unittest
 ALREADY_TESTED = False
 
 def _get_source_filename():
+    # use installed copy if available
+    tests_f = os.path.join(os.path.dirname(__file__), 'xxmodule.c')
+    if os.path.exists(tests_f):
+        return tests_f
+    # otherwise try using copy from build directory
     srcdir = sysconfig.get_config_var('srcdir')
     return os.path.join(srcdir, 'Modules', 'xxmodule.c')