fix F# testing

Try to fix the error we get in CI. No idea if this actually works, but
CI will tell us.

Author: tgummerer

testing-unit-fs-mocks/Testing.fs

@@ -17,19 +17,19 @@ type Mocks() =
                 if args.Type = "azure:storage/account:Account"
                 then ["primaryWebEndpoint", sprintf "https://%s.web.core.windows.net" args.Name :> obj]
                 else []
-            
+
             let outputs =
                 [inputKVs; nameKVs; endpointKVs]
                 |> Seq.concat
                 |> Seq.filter (fun (k, _) -> args.Type <> "azure:storage/blob:Blob" || k <> "source")
                 |> Seq.map KeyValuePair
-            let dict = outputs.ToImmutableDictionary() :> obj            
+            let dict = outputs.ToImmutableDictionary() :> obj
 
             // Default the resource ID to `{name}_id`.
-            let id = if args.Id = null then sprintf "%s_id" args.Name else args.Id 
+            let id = if args.Id = null then sprintf "%s_id" args.Name else args.Id
             Task.FromResult(struct (id, dict))
 
         member this.CallAsync(args: MockCallArgs): Task<obj> =
             // We don't use this method in this particular test suite.
             // Default to returning whatever we got as input.
-            Task.FromResult(null :> obj)
+            Task.FromResult(null :> 'a)