As of version 2017 R3 you have the ability to mark tests and standalone code files with 'In development' flag to exclude these from compilation. Thus the Compile button is extended and you could choose whether to 'Compile All' files or 'Compile' only these not marked as 'In development'.
The default action set in the Project context menu and for the Compile button is 'Compile'.
By compiling the test project you can identify compile-time errors, such as incorrect syntax, misspelled keywords, type mismatches, and incorrect method binding.
When you compile the test project Test Studio builds the project dll in the bin folder of the project.
The output of the compilation process is displayed in the Output Panel under the Compiler tab.
Press the Compile button in the ribbon to compile the entire test project.