You can use http://bip32.org/, there is a selection box in the top right hand corner of the page to choose Bitcoin Testnet. That will generate you an extended private key from a passphrase of your choosing. Note that I recommend you Do NOT use any keys generated from websites online for real world payments. But for testing this out it should be fine. Using the default passphrase it generated me:
Once you have generated an extended private key at the top, starting with
tprv, you can then select "Info: m" in the Derivation Path box, which will show you the extended public master key. In this case it generated:
Again feel free to use this for testing, but please generate an extended key correctly for the mainnet in a secure way, do not use random websites to do it!