[junit] junit result view, support stacktrace copy of single line

At the moment, in the Junit View one may (despite being able to select a single line) only copy the whole stacktrace by using `Copy Trace`. An additional option to `Copy selected line` would be nice.


Start in the JUnit View at the existing Copy Trace action and the handling of a selected stack-trace line. Add a separate Copy selected line option whose result contains only the selected line, and verify the behavior in the JUnit result view.

Written by the indexing model from the issue text.