test_ukify: use raw string for the regex

To get rid of the "invalid escape sequence" warning:

=============================== warnings summary ===============================
../src/ukify/test/test_ukify.py:876
  ../src/ukify/test/test_ukify.py:876: SyntaxWarning: invalid escape sequence '\s'
    assert re.search('Issuer: CN\s?=\s?SecureBoot signing key on host', out)

(cherry picked from commit a0485e0)

src/ukify/test/test_ukify.py

@@ -855,7 +855,7 @@ def test_key_cert_generation(tmp_path):
         '-noout',
     ], text = True)
     assert 'Certificate' in out
-    assert re.search('Issuer: CN\s?=\s?SecureBoot signing key on host', out)
+    assert re.search(r'Issuer: CN\s?=\s?SecureBoot signing key on host', out)
 
 if __name__ == '__main__':
     sys.exit(pytest.main(sys.argv))