tests/timeout: create output so tests don't trivially pass

Timeout tests rely on failed build to determine success,
so make sure these derivations (silent in particular)
don't fail regardless of timeout behavior.

tests/timeout.nix

@@ -5,6 +5,7 @@ with import ./config.nix;
   infiniteLoop = mkDerivation {
     name = "timeout";
     buildCommand = ''
+      touch $out
       echo "‘timeout’ builder entering an infinite loop"
       while true ; do echo -n .; done
     '';
@@ -13,6 +14,7 @@ with import ./config.nix;
   silent = mkDerivation {
     name = "silent";
     buildCommand = ''
+      touch $out
       sleep 60
     '';
   };
@@ -20,6 +22,7 @@ with import ./config.nix;
   closeLog = mkDerivation {
     name = "silent";
     buildCommand = ''
+      touch $out
       exec > /dev/null 2>&1
       sleep 1000000000
     '';