nixosTests.systemd-binfmt: init

This adds a simple test running GNU Hello cross-compiled for armv7l and
aarch64 inside a x86_64 VM with configured binfmt.

We already build the cross toolchains in other invocations, and building
hello itself is small.

nixos/tests/systemd-binfmt.nix

@@ -0,0 +1,24 @@
+# Teach the kernel how to run armv7l and aarch64-linux binaries,
+# and run GNU Hello for these architectures.
+import ./make-test-python.nix ({ pkgs, ... }: {
+  name = "systemd-binfmt";
+  machine = {
+    boot.binfmt.emulatedSystems = [
+      "armv7l-linux"
+      "aarch64-linux"
+    ];
+  };
+
+  testScript = let
+    helloArmv7l = pkgs.pkgsCross.armv7l-hf-multiplatform.hello;
+    helloAarch64 = pkgs.pkgsCross.aarch64-multiplatform.hello;
+  in ''
+    machine.start()
+    assert "world" in machine.succeed(
+        "${helloArmv7l}/bin/hello"
+    )
+    assert "world" in machine.succeed(
+        "${helloAarch64}/bin/hello"
+    )
+  '';
+})