internal: add slack.com to the broken auth list

Change-Id: I491418962516c71ee61eba2a4bac76967c75a5da
Reviewed-on: https://go-review.googlesource.com/15049
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/internal/token.go b/internal/token.go
index 75adbe5..db209d8 100644
--- a/internal/token.go
+++ b/internal/token.go
@@ -109,6 +109,7 @@
 	"https://test-sandbox.auth.corp.google.com",
 	"https://user.gini.net/",
 	"https://api.netatmo.net/",
+	"https://slack.com/",
 }
 
 // providerAuthHeaderWorks reports whether the OAuth2 server identified by the tokenURL