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/netxlite/tlsconn.go

@@ -0,0 +1,18 @@
+package netxlite
+
+import (
+	"crypto/tls"
+	"net"
+)
+
+// TLSConn is any tls.Conn-like structure.
+type TLSConn interface {
+	// net.Conn is the embedded conn.
+	net.Conn
+
+	// ConnectionState returns the TLS connection state.
+	ConnectionState() tls.ConnectionState
+
+	// Handshake performs the handshake.
+	Handshake() error
+}