util/httpm, all: add a test to make sure httpm is used consistently

Updates #cleanup

Change-Id: I7dbf8a02de22fc6b317ab5e29cc97792dd75352c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/httpm/httpm_test.go

@@ -0,0 +1,29 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package httpm
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestUsedConsistently(t *testing.T) {
+	cmd := exec.Command("git", "grep", "-l", "-F", "http.Method")
+	dir, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmd.Dir = filepath.Join(dir, "../..")
+	matches, _ := cmd.Output()
+	for _, fn := range strings.Split(strings.TrimSpace(string(matches)), "\n") {
+		switch fn {
+		case "util/httpm/httpm.go", "util/httpm/httpm_test.go":
+			continue
+		}
+		t.Errorf("http.MethodFoo constant used in %s; use httpm.FOO instead", fn)
+	}
+}