feat(FastAPI): add get_(errors|successes) testing HTML helpers

These functions will allow us to more easily check errors or success
messages when testing routes.

Signed-off-by: Kevin Morris <kevr@0cost.org>

aurweb/testing/html.py

@@ -1,4 +1,5 @@
 from io import StringIO
+from typing import List
 
 from lxml import etree
 
@@ -12,3 +13,13 @@ def parse_root(html: str) -> etree.Element:
     :return: etree.Element
     """
     return etree.parse(StringIO(html), parser)
+
+
+def get_errors(content: str) -> List[etree._Element]:
+    root = parse_root(content)
+    return root.xpath('//ul[@class="errorlist"]/li')
+
+
+def get_successes(content: str) -> List[etree._Element]:
+    root = parse_root(content)
+    return root.xpath('//ul[@class="success"]/li')