Address review: name the margin and explain the two additions

Split the search window into the measured duration of the test and an
explicit two second margin, and note that the range covers the oldest
second as well.

Author: iamsharduld

Lib/test/test_logging.py

@@ -6766,20 +6766,23 @@ def add_record(message: str) -> None:
         # At this point, the log file should be rotated if the rotation
         # is based on creation time but should be not if it's based on
         # modification time.  The rotated file is named after the creation
-        # time of the log file, so look back over the whole duration of the
-        # test, plus a margin: the file was created in setUp(), and the
-        # names have a resolution of one second.
-        found = False
+        # time of the log file, so search back over the whole time the test
+        # took rather than over a fixed number of seconds.
         now = datetime.datetime.now()
-        go_back = int((now - start).total_seconds()) + 2
-        for secs in range(go_back + 1):
+        test_duration = int((now - start).total_seconds())
+        # Two seconds of margin on top of that: the log file is created in
+        # setUp(), a moment before the test starts, and the name has a
+        # resolution of one second.
+        oldest = test_duration + 2
+        found = False
+        for secs in range(oldest + 1):   # inclusive of oldest
             prev = now - datetime.timedelta(seconds=secs)
             fn = self.fn + prev.strftime(".%Y-%m-%d_%H-%M-%S")
             found = os.path.exists(fn)
             if found:
                 self.rmfiles.append(fn)
                 break
-        msg = 'No rotated files found, went back %d seconds' % go_back
+        msg = 'No rotated files found, went back %d seconds' % oldest
         if not found:
             # print additional diagnostics
             dn, fn = os.path.split(self.fn)