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/engine/netx/tlsdialer/integration_test.go

@@ -16,7 +16,7 @@ func TestTLSDialerSuccess(t *testing.T) {
 	log.SetLevel(log.DebugLevel)
 	dialer := &netxlite.TLSDialer{Dialer: new(net.Dialer),
 		TLSHandshaker: &netxlite.TLSHandshakerLogger{
-			TLSHandshaker: &netxlite.TLSHandshakerStdlib{},
+			TLSHandshaker: &netxlite.TLSHandshakerConfigurable{},
 			Logger:        log.Log,
 		},
 	}