[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 <[email protected]>
Reviewed-by: Alex Rudenko <[email protected]>
Commit-Queue: Alex Rudenko <[email protected]>
Commit-Queue: Benedikt Meurer <[email protected]>

Author: bmeurer