#726 Collapse details for JUnit build problems + some report improvements

• Best to review per commit.
• Changes:
  • Consider each global/project-associated problem separately. Allows to report and display each JUnit test problem separately.
  • Introduced NonFileProblem class for global/project-associated problems. It has optional details. These are rendered as expandable elements in the report, to not clutter reports.
  • JUnit stacktrace/stdout/stderr now reported as details.
  • Test problem collector: Improved problem names/texts.
  • Test problem collector: Extended with global/project problem details.
  • Separate global/other problems with a separator line, like between projects.
  • Added header for global problems, like for project problems.
  • Only add project problems header text if applicable.

Addresses #726