Most testing tools have a 327x emulation ability that lets you interact with standard host systems like CICS or IMS applications, or any so-called green-screen applications, typical of IBM hosts. They also tend to support VT100 and 52xx screens, and compatibles. Writing test scripts for these types of applications is identical to writing the same kind of dialog scripts for web applications.
The other side of host applications consist of many different data file formats and special I/O devices like POS, ATM, etc., that very few tools support directly. When you ask about host testing it is important to provide some context so that we can be more specific in our answers.