Make credentials to use "cookie" rather than "{}" as the cookie

dynamic-imports-credentials.sub.html,
dynamic-imports-credentials-setTimeout.sub.html and credentials.sub.html
intends to test the cookie behavior based on various conditions and
ORB shouldn't be a factor here. Currently, it uses "{}" as the
cookie and let's update it to something that is not JSON to keep
the intention of the tests.

Differential Revision: https://phabricator.services.mozilla.com/D165413

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1532644
gecko-commit: 5e6a95776a530660afc2f0d540403fb48e599199
gecko-reviewers: farre, smaug

cookies/resources/set-cookie.py

@@ -28,5 +28,5 @@ def main(request, response):
         (b"Content-Type", b"application/json"),
         (b"Set-Cookie", cookie)
     ]
-    body = b"{}"
+    body = b"dummy value"
     return headers, body