[6.0] Fix a nondeterministic test failure of `testDontStackTargetPreparationForEditorFunctionality`

[ahoppen]

• Explanation: We fulfilled the allDocumentsOpened expectation directly after dispatching the notification to open LibD.swift. This left a very short window in which LibB’s preparation could finish and LibC’s expectation could start before the open notification for LibD.swift was handled, causing an unexpected preparation of LibC.
• Scope: Test-only change
• Risk: Very low, test-only change
• Testing: Ran tests
• Issue: This fixes the issue first reported as rdar://129698768 and for which I added logging in https://github.com/apple/sourcekit-lsp/pull/1478.
• Reviewer: @hamishknight on https://github.com/apple/sourcekit-lsp/pull/1499

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test Windows