refactor(netx): move dialer's mockable types in mockablex (#368)

Part of https://github.com/ooni/probe/issues/1591

internal/engine/netx/mockablex/conn.go

@@ -0,0 +1,60 @@
+package mockablex
+
+import (
+	"net"
+	"time"
+)
+
+// Conn is a mockable net.Conn.
+type Conn struct {
+	MockRead             func(b []byte) (int, error)
+	MockWrite            func(b []byte) (int, error)
+	MockClose            func() error
+	MockLocalAddr        func() net.Addr
+	MockRemoteAddr       func() net.Addr
+	MockSetDeadline      func(t time.Time) error
+	MockSetReadDeadline  func(t time.Time) error
+	MockSetWriteDeadline func(t time.Time) error
+}
+
+// Read implements net.Conn.Read
+func (c *Conn) Read(b []byte) (int, error) {
+	return c.MockRead(b)
+}
+
+// Write implements net.Conn.Write
+func (c *Conn) Write(b []byte) (int, error) {
+	return c.MockWrite(b)
+}
+
+// Close implements net.Conn.Close
+func (c *Conn) Close() error {
+	return c.MockClose()
+}
+
+// LocalAddr returns the local address
+func (c *Conn) LocalAddr() net.Addr {
+	return c.MockLocalAddr()
+}
+
+// RemoteAddr returns the remote address
+func (c *Conn) RemoteAddr() net.Addr {
+	return c.MockRemoteAddr()
+}
+
+// SetDeadline sets the connection deadline.
+func (c *Conn) SetDeadline(t time.Time) error {
+	return c.MockSetDeadline(t)
+}
+
+// SetReadDeadline sets the read deadline.
+func (c *Conn) SetReadDeadline(t time.Time) error {
+	return c.MockSetReadDeadline(t)
+}
+
+// SetWriteDeadline sets the write deadline.
+func (c *Conn) SetWriteDeadline(t time.Time) error {
+	return c.MockSetWriteDeadline(t)
+}
+
+var _ net.Conn = &Conn{}