ipn: close logger at the end of TestLocalLogLines

If any goroutine continues to use the logger in TestLocalLogLines
after the test finishes, the test panics.

The culprit for this was wireguard-go; the previous commit fixed that.
This commit adds suspenders: When the test is done, make logging calls
into no-ops.

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

ipn/loglines_test.go

@@ -26,6 +26,7 @@ func TestLocalLogLines(t *testing.T) {
 		"[v1] peer keys: %s",
 		"[v1] v%v peers: %v",
 	})
+	defer logListen.Close()
 
 	logid := func(hex byte) logtail.PublicID {
 		var ret logtail.PublicID

tstest/log.go

@@ -76,12 +76,17 @@ type LogLineTracker struct {
 	listenFor []string
 
 	mu     sync.Mutex
+	closed bool
 	seen   map[string]bool // format string => false (if not yet seen but wanted) or true (once seen)
 }
 
 // Logf logs to its underlying logger and also tracks that the given format pattern has been seen.
 func (lt *LogLineTracker) Logf(format string, args ...interface{}) {
 	lt.mu.Lock()
+	if lt.closed {
+		lt.mu.Unlock()
+		return
+	}
 	if v, ok := lt.seen[format]; ok && !v {
 		lt.seen[format] = true
 	}
@@ -101,3 +106,10 @@ func (lt *LogLineTracker) Check() []string {
 	}
 	return notSeen
 }
+
+// Close closes lt. After calling Close, calls to Logf become no-ops.
+func (lt *LogLineTracker) Close() {
+	lt.mu.Lock()
+	defer lt.mu.Unlock()
+	lt.closed = true
+}