add test helpers to check for text in the HTML output

sensors/collector/tests/helper.py

@@ -0,0 +1,12 @@
+def in_content(response, string):
+    decoded_content = response.content.decode('utf8')
+    if string not in decoded_content:
+        return False
+    return True
+
+
+def not_in_content(response, string):
+    decoded_content = response.content.decode('utf8')
+    if string in decoded_content:
+        return False
+    return True