refactor: move ErrorWrapperQUICDialer to errorsx (#420)

I needed to add some tests as integration tests due to circular
imports, but this is ~fine because we quite likely want many
integration tests in the errorsx package anyway.

Part of https://github.com/ooni/probe/issues/1505.

internal/errorsx/quic_test.go

@@ -0,0 +1,39 @@
+package errorsx
+
+import (
+	"context"
+	"crypto/tls"
+	"errors"
+	"io"
+	"testing"
+
+	"github.com/lucas-clemente/quic-go"
+	"github.com/ooni/probe-cli/v3/internal/netxmocks"
+)
+
+func TestErrorWrapperQUICDialerFailure(t *testing.T) {
+	ctx := context.Background()
+	d := &ErrorWrapperQUICDialer{Dialer: &netxmocks.QUICContextDialer{
+		MockDialContext: func(ctx context.Context, network, address string, tlsConfig *tls.Config, quicConfig *quic.Config) (quic.EarlySession, error) {
+			return nil, io.EOF
+		},
+	}}
+	sess, err := d.DialContext(
+		ctx, "udp", "www.google.com:443", &tls.Config{}, &quic.Config{})
+	if sess != nil {
+		t.Fatal("expected a nil sess here")
+	}
+	if !errors.Is(err, io.EOF) {
+		t.Fatal("expected another error here")
+	}
+	var errWrapper *ErrWrapper
+	if !errors.As(err, &errWrapper) {
+		t.Fatal("cannot cast to ErrWrapper")
+	}
+	if errWrapper.Operation != QUICHandshakeOperation {
+		t.Fatal("unexpected Operation")
+	}
+	if errWrapper.Failure != FailureEOFError {
+		t.Fatal("unexpected failure")
+	}
+}