Add a test for nix using a binder subidrectory

tests/nix/ignore-outside/README.rst

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+Check `start` works with nix
+----------------------------
+
+In this example we set a environment variable in the `start` script and check
+it works when using the nix build pack.

tests/nix/ignore-outside/binder/default.nix

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+let
+  # Pinning nixpkgs to specific release
+  # To get sha256 use "nix-prefetch-git <url> --rev <commit>"
+  commitRev="5574b6a152b1b3ae5f93ba37c4ffd1981f62bf5a";
+  nixpkgs = builtins.fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/${commitRev}.tar.gz";
+    sha256 = "1pqdddp4aiz726c7qs1dwyfzixi14shp0mbzi1jhapl9hrajfsjg";
+  };
+  pkgs = import nixpkgs { config = { allowUnfree = true; }; };
+in
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    python36Packages.notebook
+  ];
+
+  shellHook = ''
+    export NIX_PATH="nixpkgs=${nixpkgs}:."
+  '';
+}

tests/nix/ignore-outside/default.nix

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+let
+  # Pinning nixpkgs to specific release
+  # To get sha256 use "nix-prefetch-git <url> --rev <commit>"
+  commitRev="5574b6a152b1b3ae5f93ba37c4ffd1981f62bf5a";
+  nixpkgs = builtins.fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/${commitRev}.tar.gz";
+    sha256 = "1pqdddp4aiz726c7qs1dwyfzixi14shp0mbzi1jhapl9hrajfsjg";
+  };
+  pkgs = import nixpkgs { config = { allowUnfree = true; }; };
+in
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    python36Packages.numpy
+    __THIS_IS_A_SYNTAX_ERROR_THAT_SHOULD_NOT_MATTER_AS_IT_ISNT_EXECUTED__
+    python36Packages.notebook
+  ];
+
+  shellHook = ''
+    export NIX_PATH="nixpkgs=${nixpkgs}:."
+  '';
+}

tests/nix/ignore-outside/start

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+#!/bin/bash
+
+# this script should not be executed
+echo "The start script in the top level directory should not be executed"
+exit 1

tests/nix/ignore-outside/verify

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+#!/bin/bash
+set -euo pipefail
+# check that numpy isn't installed
+test -z $(pip list | grep numpy | awk '{print $1}')