refactor(tracex): do not depend on strings for event names (#777)

Rather than matching a string, match a type.

This is more robust considering future refactorings.

We're confident the names did not change in _this_ refactoring
because we're still testing the same strings in the tests.

Part of https://github.com/ooni/probe/issues/2121

internal/engine/netx/tracex/dialer.go

@@ -10,7 +10,6 @@ import (
 	"time"
 
 	"github.com/ooni/probe-cli/v3/internal/model"
-	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 // SaverDialer saves events occurring during the dial
@@ -52,14 +51,13 @@ func (d *SaverDialer) DialContext(ctx context.Context, network, address string)
 	start := time.Now()
 	conn, err := d.Dialer.DialContext(ctx, network, address)
 	stop := time.Now()
-	d.Saver.Write(Event{
+	d.Saver.Write(&EventConnectOperation{&EventValue{
 		Address:  address,
 		Duration: stop.Sub(start),
 		Err:      err,
-		Name:     netxlite.ConnectOperation,
 		Proto:    network,
 		Time:     stop,
-	})
+	}})
 	return conn, err
 }
 
@@ -124,14 +122,13 @@ func (c *saverConn) Read(p []byte) (int, error) {
 	start := time.Now()
 	count, err := c.Conn.Read(p)
 	stop := time.Now()
-	c.saver.Write(Event{
+	c.saver.Write(&EventReadOperation{&EventValue{
 		Data:     p[:count],
 		Duration: stop.Sub(start),
 		Err:      err,
 		NumBytes: count,
-		Name:     netxlite.ReadOperation,
 		Time:     stop,
-	})
+	}})
 	return count, err
 }
 
@@ -139,14 +136,13 @@ func (c *saverConn) Write(p []byte) (int, error) {
 	start := time.Now()
 	count, err := c.Conn.Write(p)
 	stop := time.Now()
-	c.saver.Write(Event{
+	c.saver.Write(&EventWriteOperation{&EventValue{
 		Data:     p[:count],
 		Duration: stop.Sub(start),
 		Err:      err,
 		NumBytes: count,
-		Name:     netxlite.WriteOperation,
 		Time:     stop,
-	})
+	}})
 	return count, err
 }