test(users/Profpatsch/writers): use testRustSimple on transitive lib

This adds a trivial test case on the transitive lib in tests and builds
it by wrapping in with testRustSimple. This should check:

* testRustSimple doesn't change the output and other packages can just
  use it as a normal dependency
* tests are built and executed

Change-Id: Ia4ea7425432b8b0da09f63054f51f0c480300aa4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2531
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/writers/default.nix

@@ -147,6 +147,7 @@ let
       python3Lib
       rustSimpleLib
       rustSimple
+      testRustSimple
       ;
    };