wgengine/magicsock: fix data race in TestNetworkSendErrors (#20261)

`TestNetworkSendErrors/network-down` causes a data race because it
tried to `tstest.Replace` the `checkNetworkDownDuringTests` global
while `wgengine.Conn.networkDown` would read from it. This patch moves
this flag into a field within the `wgengine.Conn` struct, so there’s
no chance that two tests could trample on each other.

It also renames this field to `Conn.checkNetworkUpDuringTests`,
because `Conn.networkUp` is the name of the field that gets checked.

Fixes #20260

Signed-off-by: Simon Law <sfllaw@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -425,6 +425,13 @@ type Conn struct {
 	// homeDERPGauge is the usermetric gauge for the home DERP region ID.
 	// This can be nil when [Options.Metrics] are not enabled.
 	homeDERPGauge *usermetric.Gauge
+
+	// checkNetworkUpDuringTests controls whether [Conn.networkDown]
+	// will report the value of [Conn.networkUp] while running tests.
+	//
+	// This allows tests to pass when the user's machine is offline,
+	// but allows us to still test network-down behaviour when desired.
+	checkNetworkUpDuringTests bool
 }
 
 // SetDebugLoggingEnabled controls whether spammy debug logging is enabled.
@@ -1482,14 +1489,10 @@ func (c *Conn) LocalPort() uint16 {
 
 var errNetworkDown = errors.New("magicsock: network down")
 
-// This allows tests to pass when the user's machine is offline, but allows us
-// to still test network-down behaviour when desired.
-var checkNetworkDownDuringTests = false
-
 func (c *Conn) networkDown() bool {
 	// For tests, always assume the network is up unless we're explicitly
 	// testing this behaviour.
-	if envknob.AssumeNetworkUp() || (testenv.InTest() && !checkNetworkDownDuringTests) {
+	if envknob.AssumeNetworkUp() || (testenv.InTest() && !c.checkNetworkUpDuringTests) {
 		return false
 	}
 	return !c.networkUp.Load()

wgengine/magicsock/magicsock_test.go

@@ -3386,9 +3386,8 @@ func TestNetworkSendErrors(t *testing.T) {
 			t.Skipf("skipping on %s", runtime.GOOS)
 		}
 
-		tstest.Replace(t, &checkNetworkDownDuringTests, true)
-
 		conn, reg := newTestConnAndRegistry(t)
+		conn.checkNetworkUpDuringTests = true
 
 		buffs := [][]byte{{00, 00, 00, 00, 00, 00, 00, 00}}
 		ep := &lazyEndpoint{