all: add (*testing.B).ReportAllocs() to every benchmark

This ensures that we can properly track and catch allocation
slippages that could otherwise have been missed.

Fixes #2748

net/dns/resolver/tsdns_test.go

@@ -1015,6 +1015,7 @@ func BenchmarkFull(b *testing.B) {
 
 	for _, tt := range tests {
 		b.Run(tt.name, func(b *testing.B) {
+			b.ReportAllocs()
 			for i := 0; i < b.N; i++ {
 				syncRespond(r, tt.request)
 			}

net/tsaddr/tsaddr_test.go

@@ -75,6 +75,7 @@ func TestNewContainsIPFunc(t *testing.T) {
 var sinkIP netaddr.IP
 
 func BenchmarkTailscaleServiceAddr(b *testing.B) {
+	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {
 		sinkIP = TailscaleServiceIP()
 	}

net/tstun/wrap_test.go

@@ -397,6 +397,7 @@ func TestClose(t *testing.T) {
 }
 
 func BenchmarkWrite(b *testing.B) {
+	b.ReportAllocs()
 	ftun, tun := newFakeTUN(b.Logf, true)
 	defer tun.Close()