tests/frontend/logging.py: Fix Deprecation Warning

Use raw strings for regex searches, which is the preferred way to do
regular expressions in Python.

Without this patch, currently we get the following warnings:

```
tests/frontend/logging.py:44
  /builds/BuildStream/buildstream/dist/buildstream/tests/frontend/logging.py:44: DeprecationWarning: invalid escape sequence \[
    m = re.search("\[\d\d:\d\d:\d\d\]\[\]\[\] SUCCESS Checking sources", result.stderr)

tests/frontend/logging.py:80
  /builds/BuildStream/buildstream/dist/buildstream/tests/frontend/logging.py:80: DeprecationWarning: invalid escape sequence \d
    m = re.search("\d\d:\d\d:\d\d,\d\d:\d\d:\d\d.\d{6},\d\d:\d\d:\d\d,,,SUCCESS,Checking sources", result.stderr)
```