ipn/{ipnlocal,localapi}: move most of cert.go to ipnlocal

Leave only the HTTP/auth bits in localapi.

Change-Id: I8e23fb417367f1e0e31483e2982c343ca74086ab
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/cert_test.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !ios && !android && !js
+
+package ipnlocal
+
+import "testing"
+
+func TestValidLookingCertDomain(t *testing.T) {
+	tests := []struct {
+		in   string
+		want bool
+	}{
+		{"foo.com", true},
+		{"foo..com", false},
+		{"foo/com.com", false},
+		{"NUL", false},
+		{"", false},
+		{"foo\\bar.com", false},
+		{"foo\x00bar.com", false},
+	}
+	for _, tt := range tests {
+		if got := validLookingCertDomain(tt.in); got != tt.want {
+			t.Errorf("validLookingCertDomain(%q) = %v, want %v", tt.in, got, tt.want)
+		}
+	}
+}