main: check that code is formatted in unit test

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

format_test.go

@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2021 WireGuard LLC. All Rights Reserved.
+ */
+package main
+
+import (
+	"bytes"
+	"go/format"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"sync"
+	"testing"
+)
+
+func TestFormatting(t *testing.T) {
+	var wg sync.WaitGroup
+	filepath.WalkDir(".", func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			t.Errorf("unable to walk %s: %v", path, err)
+			return nil
+		}
+		if d.IsDir() || filepath.Ext(path) != ".go" {
+			return nil
+		}
+		wg.Add(1)
+		go func(path string) {
+			defer wg.Done()
+			src, err := os.ReadFile(path)
+			if err != nil {
+				t.Errorf("unable to read %s: %v", path, err)
+				return
+			}
+			formatted, err := format.Source(src)
+			if err != nil {
+				t.Errorf("unable to format %s: %v", path, err)
+				return
+			}
+			if !bytes.Equal(src, formatted) {
+				t.Errorf("unformatted code: %s", path)
+			}
+		}(path)
+		return nil
+	})
+	wg.Wait()
+}