What library or wallet can be used to test segwit transactions on testnet? Is there any code snippets / wallets to send segwit transactions on the testnet at the moment? 

I'm more looking for a code snippet or any library to support this to do some tests and make them public as a testing suit for wallet services.
bitcoin core supports this

addwitnessaddress "address"


then compose your transaction and send it, you can do it all via RPC calls, so any programming language that can do rpc calls will work (python, php, go, etc. etc)

help addwitnessaddress


addwitnessaddress "address"

Add a witness address for a script (with pubkey or redeemscript known).
It returns the witness script.

Arguments:
1. "address"       (string, required) An address known to the wallet

Result:
"witnessaddress",  (string) The value of the new address (P2SH of witness script).
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