nixos/borgbackup: add test

We had problems to get borg's own test suite running.
This test is intended to perform a quick smoke test to see whether we
have missed not any important dependency necessary to create backups
with borg.

tested with:

$ nix-build nixos/release.nix -A tests.borgbackup.x86_64-linux

nixos/tests/borgbackup.nix

@@ -0,0 +1,21 @@
+import ./make-test.nix ({ pkgs, ...}: {
+  name = "borgbackup";
+  meta = with pkgs.stdenv.lib.maintainers; {
+    maintainers = [ mic92 ];
+  };
+
+  nodes = {
+    machine = { config, pkgs, ... }: {
+      environment.systemPackages = [ pkgs.borgbackup ];
+    };
+  };
+
+  testScript = ''
+    my $borg = "BORG_PASSPHRASE=supersecret borg";
+    $machine->succeed("$borg init --encryption=repokey /tmp/backup");
+    $machine->succeed("mkdir /tmp/data/ && echo 'data' >/tmp/data/file");
+    $machine->succeed("$borg create --stats /tmp/backup::test /tmp/data");
+    $machine->succeed("$borg extract /tmp/backup::test");
+    $machine->succeed('c=$(cat data/file) && echo "c = $c" >&2 && [[ "$c" == "data" ]]');
+  '';
+})