Use "output" as format argument

Nearly everywhere else the query argument for output format is "t";
but here "t" is used for operations, and "output" is used for output
format.

The test suite will fail without this.

src/allmydata/web/check_results.py

@@ -211,6 +211,8 @@ class ResultsBase(object):
 
 class LiteralCheckResultsRenderer(MultiFormatResource, ResultsBase):
 
+    formatArgument = "output"
+
     def __init__(self, client):
         super(LiteralCheckResultsRenderer, self).__init__()
         self.client = client