test/sandbox: guard on testtool tag

This tool should not show up when building hakurei normally.

Signed-off-by: Ophestra <cat@gensokyo.uk>

test/sandbox/assert.go

@@ -1,3 +1,5 @@
+//go:build testtool
+
 /*
 Package sandbox provides utilities for checking sandbox outcome.
 

test/sandbox/assert_test.go

@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox
 
 import (

test/sandbox/fs.go

@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox
 
 import (

test/sandbox/fs_test.go

@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox_test
 
 import (

test/sandbox/mount.go

@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox
 
 /*

test/sandbox/mount_test.go

@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox_test
 
 import (

test/sandbox/ptrace.go

@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox
 
 import (

test/sandbox/seccomp.go

@@ -1,3 +1,5 @@
+//go:build testtool
+
 package sandbox
 
 import (

test/sandbox/tool/main.go

@@ -1,3 +1,5 @@
+//go:build testtool
+
 package main
 
 import (

test/sandbox/tool/package.nix

@@ -17,6 +17,8 @@ buildGoModule rec {
   };
   vendorHash = null;
 
+  tags = [ "testtool" ];
+
   buildInputs = [ util-linux ];
   nativeBuildInputs = [ pkg-config ];