Writing unit tests _is_ part of the interactive development style in Smalltalk. In fact TDD arose out of a lot of (ex)-Smalltalk developers. When you have an environment as mikelevins describes, where the debugger allows you to author-as-you-go, you can start by writing a test and fill in the implementation and have your test go green, all in one very tight loop.
You actually can run a bunch of unit tests in Smalltalk with a single-click, get results, and be dropped into a full authoring environment when tests fail, with the bonus that you can just fix the code and resume execution to have the test go green. Modern Java IDEs can probably do this (I've no experience with Java), but you've to understand that this was possible in Smalltalk several decades ago.
You actually can run a bunch of unit tests in Smalltalk with a single-click, get results, and be dropped into a full authoring environment when tests fail, with the bonus that you can just fix the code and resume execution to have the test go green. Modern Java IDEs can probably do this (I've no experience with Java), but you've to understand that this was possible in Smalltalk several decades ago.