Otherwise closing and re-opening a file will stop it from updating.
There is a chance that a file is modified while it was closed, but I don't think there's a way to handle this (since we don't get proper edits).
Fixes #5878

When a document is closed, isClosed is set to true and re-opening can return a new `TextDocument` instance.
Although no tests failed with the previous code, I suspect that's because VS Code only cleans up closed TextDocuments periodically and we don't have a way to force it 😔

These were effectively the same now we're clearing all positions if any becomes invalid.

When we are setting a test up, if there are no open files, or only the one we are going to open, we can skip this (rather slow) work.

DanTup deleted the persist-test-trackers branch