examples: Add testing/crash-before-switch-root

Can be used as a single point of failure (hah!)

examples/testing/crash-before-switch-root/README.md

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+`crash-before-switch-root`
+==========================
+
+What does this test?
+--------------------
+
+A simple known method to crash the system at boot.
+
+This can be used to check changes to the errors handling.
+
+
+Why is this scary?
+------------------
+
+Not scary at all. Only pretty useless in daily use!
+
+
+How is success defined?
+-----------------------
+
+The boot process should proceed normally (splash, etc), but be
+interrupted before switch root.
+
+The crashing happens *after* mounting /mnt, but *before* switching root.
+
+
+Running
+-------
+
+Assuming you are `cd`'d into the root of a Mobile NixOS checkout:
+
+```
+nix-build ./examples/testing/crash-before-switch-root && ./result
+```
+
+This will build for qemu-x86_64 by default.
+
+As always, be mindful of your `NIX_PATH`.

examples/testing/crash-before-switch-root/configuration.nix

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+{ config, lib, pkgs, ... }:
+
+{
+  mobile.boot.stage-1.tasks = [ ./crash.rb ];
+
+  mobile.boot.stage-1.bootConfig = {
+    log.level = lib.mkForce "INFO";
+  };
+}

examples/testing/crash-before-switch-root/crash.rb

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+class Tasks::Crash < SingletonTask
+  def initialize()
+    # Runs before SwitchRoot
+    Targets[:SwitchRoot].add_dependency(:Task, self)
+    # And after /mnt is available
+    add_dependency(:Mount, "/mnt")
+  end
+
+  def run()
+    raise "This is an exception from init!"
+  end
+end

examples/testing/crash-before-switch-root/default.nix

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+{ device ? "qemu-x86_64" }:
+let
+  system-build = import ../../../. {
+    inherit device;
+    configuration = [ { imports = [
+      ../../hello/configuration.nix
+      ./configuration.nix
+    ]; } ];
+  };
+in
+  system-build.build.default