all: use tstest.Replace more

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

ipn/ipnlocal/local_test.go

@@ -497,8 +497,7 @@ func (panicOnUseTransport) RoundTrip(*http.Request) (*http.Response, error) {
 
 // Issue 1573: don't generate a machine key if we don't want to be running.
 func TestLazyMachineKeyGeneration(t *testing.T) {
-	defer func(old func() bool) { panicOnMachineKeyGeneration = old }(panicOnMachineKeyGeneration)
-	panicOnMachineKeyGeneration = func() bool { return true }
+	tstest.Replace(t, &panicOnMachineKeyGeneration, func() bool { return true })
 
 	var logf logger.Logf = logger.Discard
 	store := new(mem.Store)

ipn/localapi/localapi_test.go

@@ -14,6 +14,7 @@ import (
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn/ipnlocal"
+	"tailscale.com/tstest"
 )
 
 func TestValidHost(t *testing.T) {
@@ -42,10 +43,7 @@ func TestValidHost(t *testing.T) {
 }
 
 func TestSetPushDeviceToken(t *testing.T) {
-	validLocalHostForTesting = true
-	defer func() {
-		validLocalHostForTesting = false
-	}()
+	tstest.Replace(t, &validLocalHostForTesting, true)
 
 	h := &Handler{
 		PermitWrite: true,