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>

types/logger/logger.go

@@ -179,3 +179,16 @@ func (fn ArgWriter) Format(f fmt.State, _ rune) {
 }
 
 var argBufioPool = &sync.Pool{New: func() interface{} { return bufio.NewWriterSize(ioutil.Discard, 1024) }}
+
+// Filtered returns a Logf that silently swallows some log lines.
+// Each inbound format and args is evaluated and printed to a string s.
+// The original format and args are passed to logf if and only if allow(s) returns true.
+func Filtered(logf Logf, allow func(s string) bool) Logf {
+	return func(format string, args ...interface{}) {
+		msg := fmt.Sprintf(format, args...)
+		if !allow(msg) {
+			return
+		}
+		logf(format, args...)
+	}
+}