This continues on the coverage API I started a few years ago. It adds
initial integration where a "Show Test Coverage" tree item is shown in
the Test Results view, which then opens a dedicated Test Coverage view.
The Test Coverage view is a fairly basic tree view following the draft
design, with further improvements to come.
The 'bars' widget is also built in a reusable way such that it can be
integrated into the explorer, as this was a popular ask both inside and
outside the team.
For #123713.
e0ef3fcd18