Workflow steps can be tested using a process definition and instance.  Due to dependencies, workflow steps must be tested within a workflow.  As developers, if a test needs to be tested independently, then select a step and use the following menu:

The step-testing UI looks as below. Provide the step input and click the Test button. 

The step is executed, and the confirmation message is displayed in the top right-end corner. 

If you are a developer who wants to debug the C# step code, attach the Microsoft Visual Studio debugger to the “w3p.exe” process. Once the test button is pressed, the debugger will jump to the step code and break at the selected break point.