tests: fix Anomalous backslash in string

[pylint] Anomalous backslash in string: '\S'. String constant might be
missing an r prefix.

slapos/rebootstrap/tests.py

@@ -107,9 +107,9 @@ def test_suite():
   kwargs = dict(setUp=setUp,
       tearDown=zc.buildout.testing.buildoutTearDown,
       checker=renormalizing.RENormalizing([
-                        (re.compile('--prefix=\S+sample-buildout'),
+                        (re.compile(r'--prefix=\S+sample-buildout'),
                          '--prefix=/sample_buildout'),
-                        (re.compile('\s/\S+sample-buildout'),
+                        (re.compile(r'\s/\S+sample-buildout'),
                          ' /sample_buildout'),
                         (re.compile(sys.executable),
                          '/system_python'),