test/sandbox: assert filesystem json

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

test/sandbox/assert_test.go

@@ -1,6 +1,32 @@
 package sandbox
 
+import (
+	"encoding/json"
+	"os"
+	"path"
+	"testing"
+)
+
 type F func(format string, v ...any)
 
 func SwapPrint(f F) (old F) { old = printfFunc; printfFunc = f; return }
 func SwapFatal(f F) (old F) { old = fatalfFunc; fatalfFunc = f; return }
+
+func MustWantFile(t *testing.T, v any) (wantFile string) {
+	wantFile = path.Join(t.TempDir(), "want.json")
+	if f, err := os.OpenFile(wantFile, os.O_CREATE|os.O_WRONLY, 0400); err != nil {
+		t.Fatalf("cannot create %q: %v", wantFile, err)
+	} else if err = json.NewEncoder(f).Encode(v); err != nil {
+		t.Fatalf("cannot encode to %q: %v", wantFile, err)
+	} else if err = f.Close(); err != nil {
+		t.Fatalf("cannot close %q: %v", wantFile, err)
+	}
+
+	t.Cleanup(func() {
+		if err := os.Remove(wantFile); err != nil {
+			t.Fatalf("cannot remove %q: %v", wantFile, err)
+		}
+	})
+
+	return
+}