Update Documentation for running single file test

[zeroryu]

Currently, you have to make sure that the platformio-ide terminal settings on working directory is set to "Project" level. the {relative_path} will not run if you set it other ways.

Another option, would it make sense to provide just the {filename} on top of the {relative_path}? that way the ide will be more flexible to use with

[nolanpro]

@zeroryu Thanks, I updated the documentation in a PR. Separating the filename from the relative path is definitely something worth looking into.