fix(netxlite): improve TLS auto-configuration (#409)

Auto-configure every relevant TLS field as close as possible to
where it's actually used.

As a side effect, add support for mocking the creation of a TLS
connection, which should possibly be useful for uTLS?

Work that is part of https://github.com/ooni/probe/issues/1505

internal/netxmocks/tlsconn.go

@@ -0,0 +1,25 @@
+package netxmocks
+
+import "crypto/tls"
+
+// TLSConn allows to mock netxlite.TLSConn.
+type TLSConn struct {
+	// Conn is the embedded mockable Conn.
+	Conn
+
+	// MockConnectionState allows to mock the ConnectionState method.
+	MockConnectionState func() tls.ConnectionState
+
+	// MockHandshake allows to mock the Handshake method.
+	MockHandshake func() error
+}
+
+// ConnectionState calls MockConnectionState.
+func (c *TLSConn) ConnectionState() tls.ConnectionState {
+	return c.MockConnectionState()
+}
+
+// Handshake calls MockHandshake.
+func (c *TLSConn) Handshake() error {
+	return c.MockHandshake()
+}