I was wondering how testnet addresses like https://testnet.blockchain.info/address/myaj43o2wt34j31ej5pmP6htCHFafAKoeP are created? Also is there a script to create them?
this address is a standard P2PKH testnet address, and can be derived from a typical priv/public keypair, by adding the testnet prefix. There is a page you can play with - in Step 4 you would change the default prefix "00" to "6F", and get a corresponding address. The composition of such an address and further details are again in the wiki .