Fix format of milliseconds in pytest output (#10836)

## Problem

The timestamp prefix of pytest log lines contains milliseconds without
leading zeros, so values of milliseconds less than 100 printed
incorrectly.

For example:
```
2025-02-15 12:02:51.997 INFO [_internal.py:97] 127.0.0.1 - - ...
2025-02-15 12:02:52.4   INFO [_internal.py:97] 127.0.0.1 - - ...
2025-02-15 12:02:52.9   INFO [_internal.py:97] 127.0.0.1 - - ...
2025-02-15 12:02:52.23  INFO [_internal.py:97] 127.0.0.1 - - ...
```

## Summary of changes
Fix log_format for pytest so that milliseconds are printed with leading
zeros.

Author: alexanderlaw

pytest.ini

@@ -11,7 +11,7 @@ markers =
 testpaths =
     test_runner
 minversion = 6.0
-log_format = %(asctime)s.%(msecs)-3d %(levelname)s [%(filename)s:%(lineno)d] %(message)s
+log_format = %(asctime)s.%(msecs)03d %(levelname)s [%(filename)s:%(lineno)d] %(message)s
 log_date_format = %Y-%m-%d %H:%M:%S
 log_cli = true
 timeout = 300