go.mod: restore staticcheck module, make it stick around, go mod tidy

It kept coming & going as different people ran go mod tidy and others
ran staticcheck.

Make it stop going away with go mod tidy by adding a dep to it.

tstest/staticcheck/staticcheck.go

@@ -0,0 +1,12 @@
+// Copyright (c) 2020 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.
+
+// This file exists just so go mod tidy won't remove
+// staticcheck's module from our go.mod.
+
+package tstest
+
+import (
+	_ "honnef.co/go/tools/staticcheck"
+)