Add basic flake tests

tests/config.nix.in

@@ -0,0 +1,20 @@
+with import <nix/config.nix>;
+
+rec {
+  inherit shell;
+
+  path = coreutils;
+
+  system = "@system@";
+
+  shared = builtins.getEnv "_NIX_TEST_SHARED";
+
+  mkDerivation = args:
+    derivation ({
+      inherit system;
+      builder = shell;
+      args = ["-e" args.builder or (builtins.toFile "builder.sh" "if [ -e .attrs.sh ]; then source .attrs.sh; fi; eval \"$buildCommand\"")];
+      PATH = path;
+    } // removeAttrs args ["builder" "meta"])
+    // { meta = args.meta or {}; };
+}