[test-runner] Move test case processing beyond the multi-process boundary.

test/cctest/testcfg.py

Index: test/cctest/testcfg.py
diff --git a/test/cctest/testcfg.py b/test/cctest/testcfg.py
index bd93450a97e87e3b184174932fb56cd832dbdfd4..d28ef7d0044d48abd87864f64b7730d2e47731fb 100644
--- a/test/cctest/testcfg.py
+++ b/test/cctest/testcfg.py
@@ -44,6 +44,10 @@ class CcTestSuite(testsuite.TestSuite):
       build_dir = "out"
     self.serdes_dir = os.path.normpath(
         os.path.join(root, "..", "..", build_dir, ".serdes"))
+
+  def SetupWorkingDirectory(self):
+    # This is only called once per machine, while init above is called once per
+    # process.
     if os.path.exists(self.serdes_dir):
       shutil.rmtree(self.serdes_dir, True)
     os.makedirs(self.serdes_dir)