wgengine: quiet some wireguard-go logging

The log lines that wireguard-go prints as it starts
and stops its worker routines are mostly noise.
They also happen after other work is completed,
which causes failures in some of the log testing packages.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

wgengine/userspace.go

@@ -247,9 +247,15 @@ func newUserspaceEngineAdvanced(conf EngineConfig) (_ Engine, reterr error) {
 	}
 	e.magicConn.SetNetworkUp(e.linkState.AnyInterfaceUp())
 
+	// wireguard-go logs as it starts and stops routines.
+	// Silence those; there are a lot of them, and they're just noise.
+	allowLogf := func(s string) bool {
+		return !strings.HasPrefix(s, "Routine:")
+	}
+	filtered := logger.Filtered(logf, allowLogf)
 	// flags==0 because logf is already nested in another logger.
 	// The outer one can display the preferred log prefixes, etc.
-	dlog := logger.StdLogger(logf)
+	dlog := logger.StdLogger(filtered)
 	logger := device.Logger{
 		Debug: dlog,
 		Info:  dlog,