Support for real-time test reporting

[orchestr7]

Right now save-cli is writing test report after all logic is already done. We keep all results in the memory and only after all tests are processed save provides them to a reporter that is writing results.

We have two variants here:

1. in real time write results to the file (one test - one file)
2. append results to some pipe/file in a real time

That will be useful for save-cloud

[petertrr]

We keep all results in the memory and only after all tests are processed save provides them to a reporter that is writing results.

That's not true. Reporter already has a number of 'lifecycle listeners', e.g. onEvent(event: TestResult). PlainTextReporter is writing all data in real time, as does JSON reporter. However, for JSON reporter report becomes properly formatted only after the execution is finished, because JSON is a structured text format, unlike plain text.