all: rename variables with lowercase-l/uppercase-I

See http://go/no-ell

Signed-off-by: Alex Chan <alexc@tailscale.com>

Updates #cleanup

Change-Id: I8c976b51ce7a60f06315048b1920516129cc1d5d

net/speedtest/speedtest_test.go

@@ -21,13 +21,13 @@ func TestDownload(t *testing.T) {
 	flakytest.Mark(t, "https://github.com/tailscale/tailscale/issues/17338")
 
 	// start a listener and find the port where the server will be listening.
-	l, err := net.Listen("tcp", ":0")
+	ln, err := net.Listen("tcp", ":0")
 	if err != nil {
 		t.Fatal(err)
 	}
-	t.Cleanup(func() { l.Close() })
+	t.Cleanup(func() { ln.Close() })
 
-	serverIP := l.Addr().String()
+	serverIP := ln.Addr().String()
 	t.Log("server IP found:", serverIP)
 
 	type state struct {
@@ -40,7 +40,7 @@ func TestDownload(t *testing.T) {
 	stateChan := make(chan state, 1)
 
 	go func() {
-		err := Serve(l)
+		err := Serve(ln)
 		stateChan <- state{err: err}
 	}()
 
@@ -84,7 +84,7 @@ func TestDownload(t *testing.T) {
 	})
 
 	// causes the server goroutine to finish
-	l.Close()
+	ln.Close()
 
 	testState := <-stateChan
 	if testState.err != nil {