envknob: add GOOS func

Centralize the fake GOOS stuff, start to use it more. To be used more
in the future.

Change-Id: Iabacfbeaf5fca0b53bf4d5dbcdc0367f05a205f9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

envknob/envknob_testable.go

@@ -0,0 +1,24 @@
+// Copyright (c) 2022 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 !ts_not_in_tests
+
+package envknob
+
+import "runtime"
+
+// GOOS reports the effective runtime.GOOS to run as.
+//
+// In practice this returns just runtime.GOOS, unless overridden by
+// test TS_DEBUG_FAKE_GOOS.
+//
+// This allows changing OS-specific stuff like the IPN server behavior
+// for tests so we can e.g. test Windows-specific behaviors on Linux.
+// This isn't universally used.
+func GOOS() string {
+	if v := String("TS_DEBUG_FAKE_GOOS"); v != "" {
+		return v
+	}
+	return runtime.GOOS
+}