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)
```

Specify flags at the start of the expression as per the recommendation
of the standard library.

Without this patch, we currently get the following warning:

```
tests/examples/junctions.py::test_open_cross_junction_workspace
  /builds/BuildStream/buildstream/dist/buildstream/buildstream/utils.py:213: DeprecationWarning: Flags not at the start of the expression '\\/[^/]*\\Z(?ms)'
    regexer = re.compile(expression)
```