Issue #27076: Doc, comment and test function name spelling fixes

Most fixes to Doc/ and Lib/ directories by Ville Skyttä.
diff --git a/Lib/unittest/result.py b/Lib/unittest/result.py
index 2cc17d7..8af972f 100644
--- a/Lib/unittest/result.py
+++ b/Lib/unittest/result.py
@@ -132,7 +132,7 @@
         self.skipped.append((test, reason))
 
     def addExpectedFailure(self, test, err):
-        """Called when an expected failure/error occured."""
+        """Called when an expected failure/error occurred."""
         self.expectedFailures.append(
             (test, self._exc_info_to_string(err, test)))