Issue #10512: close the log file in cgi when running tests.

Thanks to Nadeem Vawda for the find and an initial fix.
diff --git a/Lib/test/test_cgi.py b/Lib/test/test_cgi.py
index 3b6f59b..985de11 100644
--- a/Lib/test/test_cgi.py
+++ b/Lib/test/test_cgi.py
@@ -155,6 +155,7 @@
             cgi.logfp = None
             cgi.logfile = "/dev/null"
             cgi.initlog("%s", "Testing log 3")
+            self.addCleanup(cgi.logfp.close)
             cgi.log("Testing log 4")
 
     def test_fieldstorage_readline(self):