Add bdd test for page not containing plaintext.

You can now use 'the page should not contain the plaintext ...' in a
feature test.

tests/integration/features/conftest.py

@@ -366,6 +366,14 @@ def check_contents_plain(quteproc, text):
     assert text in content
 
 
+@bdd.then(bdd.parsers.parse('the page should not contain the plaintext '
+                            '"{text}"'))
+def check_contents_plain(quteproc, text):
+    """Check the current page's content based on a substring."""
+    content = quteproc.get_content().strip()
+    assert text not in content
+
+
 @bdd.then(bdd.parsers.parse('the json on the page should be:\n{text}'))
 def check_contents_json(quteproc, text):
     """Check the current page's content as json."""