Allow to clear named TestResult.Section.output buffer

Implementation of 4422418 added a nice feature that client code can use to put some data into TestResult object in realtime, while test is still running. Harness has also changed implementation of some UI components to show those updated dynamically.

This request asks to add possibility to re-create TestResult.Section.output or clear all strings from there and put new contents (replace).

Usecase is following:
1) Our tests produce log messages from several components and we show these in TestResult.Section.output(@name="logs").
2) Due to distributed nature of tests, some of log messages may arrive earlier than others, some may arrive at the very end of the test. In order to show them in appropriate order, we added timestamps for all of logs and RESORT all entries when we receive test result.
3) Because of above we can't push out logs in realtime to TestResult - because we'll not be able to refresh them once we collect them all and resort.

What we'd like to implement is:
1) Push all logs into TestResult.Section.output once they arrive
2) When test result arrives - we'd like to
 a) clear named output buffer completely
 b) resort our log messages
 c) put them again into the same output.

So, there's a need to implement something like ".clearOutput(String outputName)" or ".deleteOutputData(String name)" in TestResult.Section