With Swift 6.1 the extractor will start to extract files outside of the test
directory. These files and their elements we do not want to see in our tests.
* do not mention any more that one might make analysis happen on Linux with
advanced setup
* say that outright Swift analysis is only supported on macOS, not just
autobuild.
* emit the error diagnostics even for traced builds, not only for autobuilds
(by using a dummy `extractor` executable).
We are no longer bound to the platform-specific directories,
so simplify the test organization.
If you don't want this change, just skip merging this PR. It's purely optional.
