The paper Two Little Languages: SchemeUnit and SchemeQL describes the design rationale and implementation of SchemeUnit.
There are copious comments in the source code. The file
example.ss, provided as part of SchemeUnit, is a
good example of a reasonably complex test suite.
Good references for the Extreme Programming methodology include
JUnit, the Java unit test framework.
Most of the code at Schematics has extensive test suites written using SchemeUnit.