refactor(netxlite): restructure dialer tests (#486)

While there, add one more test checking for whether the internal
CA bundle we use can actually be loaded.

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

internal/netxlite/legacy.go

@@ -10,7 +10,7 @@ import (
 
 // These vars export internal names to legacy ooni/probe-cli code.
 var (
-	DefaultDialer        = defaultDialer
+	DefaultDialer        = &dialerSystem{}
 	DefaultTLSHandshaker = defaultTLSHandshaker
 	NewConnUTLS          = newConnUTLS
 )