Add a test for the gc with CA derivations

Also add a small architecture to easily run CA-enabled tests

tests/config.nix.in

@@ -1,3 +1,12 @@
+let
+  contentAddressedByDefault = builtins.getEnv "NIX_TESTS_CA_BY_DEFAULT" == "1";
+  caArgs = if contentAddressedByDefault then {
+    __contentAddressed = true;
+    outputHashMode = "recursive";
+    outputHashAlgo = "sha256";
+  } else {};
+in
+
 rec {
   shell = "@bash@";
 
@@ -15,4 +24,4 @@ rec {
       PATH = path;
     } // removeAttrs args ["builder" "meta"])
     // { meta = args.meta or {}; };
-}
+} // caArgs