[Feature request][Test explorer] List tests "by file", rather than "by tree"

[diogo-rossi]

Currently, we can view tests listed "as tree" or "as list"

But, viewing "by tree" list folders and sub-folders, that may require additional clicks to show the files, which is unnecessary when there is only one project, and one folder with tests.

This request is to make possible list test "by file", like it is done in the other VSCode python test adapter.