Actually, forget that (!). The following patch seems to work:

Index: Lib/multiprocessing/forking.py
===================================================================
--- Lib/multiprocessing/forking.py	(révision 88224)
+++ Lib/multiprocessing/forking.py	(copie de travail)
@@ -474,7 +474,8 @@
             else:
                 dirs = [os.path.dirname(main_path)]
 
-            assert main_name not in sys.modules, main_name
+            if main_name != '__main__':
+                assert main_name not in sys.modules, main_name
             file, path_name, etc = imp.find_module(main_name, dirs)
             try:
                 # We would like to do "imp.load_module('__main__', ...)"
Index: Lib/test/__main__.py
===================================================================
--- Lib/test/__main__.py	(révision 88224)
+++ Lib/test/__main__.py	(copie de travail)
@@ -1,13 +1,16 @@
 from test import regrtest, support
 
+# Issue #10845: avoid executing toplevel code if imported by multiprocessing
+# (which is smart enough to import the module under another name).
+if __name__ == "__main__":
 
-TEMPDIR, TESTCWD = regrtest._make_temp_dir_for_build(regrtest.TEMPDIR)
-regrtest.TEMPDIR = TEMPDIR
-regrtest.TESTCWD = TESTCWD
+    TEMPDIR, TESTCWD = regrtest._make_temp_dir_for_build(regrtest.TEMPDIR)
+    regrtest.TEMPDIR = TEMPDIR
+    regrtest.TESTCWD = TESTCWD
 
-# Run the tests in a context manager that temporary changes the CWD to a
-# temporary and writable directory. If it's not possible to create or
-# change the CWD, the original CWD will be used. The original CWD is
-# available from support.SAVEDCWD.
-with support.temp_cwd(TESTCWD, quiet=True):
-    regrtest.main()
+    # Run the tests in a context manager that temporary changes the CWD to a
+    # temporary and writable directory. If it's not possible to create or
+    # change the CWD, the original CWD will be used. The original CWD is
+    # available from support.SAVEDCWD.
+    with support.temp_cwd(TESTCWD, quiet=True):
+        regrtest.main()