A port of Kent Beck's SUnit is delivered with GNU Smalltalk, a brief overview could also be found in the info-documentation (Node SUnit, under Packages).
Additional but squeak-centric information can be found in Eng-Art8-SUnit-V1.pdf
To use SUnit in GNU Smalltalk, you'll have to file in SUnit, if not done already.
PackageLoader fileInPackage: 'SUnit'!
You'll maybe find
MyTest buildSuiteFromLocalSelectors run printNl!
useful, as it creates a suite containing all tests of MyTest? (methods beginning with test).
If the test fails, you should evaluate
(MyTest new) setUp; testMyFailingTest; tearDown!
as it gives the reason in the displayed walkback.