util/testenv: add new package to hold InTest

Removes duplicated code.

Updates #cleanup

Signed-off-by: Maisem Ali <maisem@tailscale.com>

util/testenv/testenv.go

@@ -0,0 +1,21 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package testenv provides utility functions for tests. It does not depend on
+// the `testing` package to allow usage in non-test code.
+package testenv
+
+import (
+	"flag"
+
+	"tailscale.com/types/lazy"
+)
+
+var lazyInTest lazy.SyncValue[bool]
+
+// InTest reports whether the current binary is a test binary.
+func InTest() bool {
+	return lazyInTest.Get(func() bool {
+		return flag.Lookup("test.v") != nil
+	})
+}