Writing a test harness is a difficult and repetitive programming task, and the lack of tool support for customized automated testing is an obstacle to the adoption of more sophisticated testing in industry. This paper presents TSTL, the Template Scripting Testing Language, which allows users to specify the general form of valid tests for a system in a simple but expressive language, and tools to support testing based on a TSTL definition. TSTL is a minimalist template- based domain-specific language, using the source language of the Software Under Test (SUT) to support most operations, but adding declarative idioms for testing. TSTL compiles to a common testing interface that hides the details of the SUT and provides support for logging, code coverage, delta debugging, and other core testing functionality, making it easy to write universal testing tools such as random testers or model checkers that apply to all TSTL-defined harnesses. TSTL is currently available for Python, but easily adapted to other languages as well.