[sources] Add tests for AutomaticFileSystemWorkspaceBinding.

Drive-by-fix: Use `sinon.assert` in `AutomaticFileSystemManager` tests
as well to produce more readable error messages.

Bug: 395562934
Change-Id: I8c972cf2734c0fddf9661f323005b0cde77a21a2
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6446433
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Alex Rudenko <alexrudenko@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>

Author: bmeurer