Blockchain.info allows you to look at the raw hex of a transaction. But it doesn't have testnet functionality. Blockexplorer allows you to look at testnet, but I can't find a way to show the hex dump.
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Maybe too late, but here's an example of using Blockr.io on testnet with raw hex dump:
Here is a functioning Testnet Tx Hex decoder https://live.blockcypher.com/btc-testnet/decodetx/
You can also get raw transaction on bitaps.com testent explorer