Depends what you call a "real test" - if playing the best is the only real test, then all cricket not involving Australia is pointless.
In 2004 England won 3-0 in The Caribbean, something that apparently no-one has ever done before (yes - not even Australia could manage it!). They then went through the summer with a 100% record, something I don't think means much more than winning 3 and drawing 3 would have done 8 years ago - results are the way the game is going ATM. Yet England played well enough to show that they were convincingly better than two other mid-table teams, which in itself is a relief after drawing at home to Pakistan, in New Zealand and at home to India in the 3 previous years, all series which should have been won.
England's true test IMO is in the current series - I don't think they have a real chance of beating Australia and nor do many people other than the "bring on the Aussies!" brigade.
If they win in South Africa, though, they will have achieved something quite special.
But as I've come to realise recently (thanks in no small part to wpdavid) - "it's a very long time since England have endured a successful run against top-of-the-tree quality opposition". It is nice, really, to simply be beating convincingly the other sides in mid-table, and as I say, that is the real test at present.
Seriously, when was the last time England were the best side in The World? Briefly in the 1950s, yes, but otherwise before WW1 was the only time.