Add a config option for hiding folder hierarchy from tree viewer

[riclucio]

When Test Explorer is filtered to show only tests from the current file (@doc), and it's configured as "View as tree", the explorer shows the entire path down to the test file working on, which seems useless.
In this screenshot, the part in red will always be expanded, polluting the viewer:

Assuming there are legit use cases for showing the entire path in the viewer even when focusing on a single test file, I'm proposing that a new setting is added (or another option say "View as simplified tree" or something, that hides all the nodes corresponding to folders from the tree viewer, showing only the test file and everything below it (describe's and test's).

[connectdotz]

Is this similar to #871?