lib: Fix system parsing, and use for doubles lists

The old hard-coded lists are now used to test system parsing.

In the process, make an `assertTrue` in release lib for eval tests; also
use it in release-cross

pkgs/top-level/release-lib.nix

@@ -46,6 +46,11 @@ rec {
   pkgs_x86_64_cygwin = allPackages { system = "x86_64-cygwin"; };
 
 
+  assertTrue = bool:
+    if bool
+    then pkgs.runCommand "evaluated-to-true" {} "touch $out"
+    else pkgs.runCommand "evaluated-to-false" {} "false";
+
   /* The working or failing mails for cross builds will be sent only to
      the following maintainers, as most package maintainers will not be
      interested in the result of cross building a package. */