Expected results editor file sync issues

[donaldoakes]

Expected result fragments served by resultFs can get out-of-sync with full suite expected result YAML file editors.

Scenarios:

• In Test Explorer open result fragment diff editor by clicking Compare Result Fragments icon. Make a change but don't save. Open the full diff editor for the suite by clicking Compare Result Files icon. Changes are not reflected.
• Open full suite diff editor. Open fragment diff editor. Change in fragment (no save) and switch back to full, and change is not reflected.
• Open fragment diff editor. Open full suite diff editor. Change in full suite editor (no save) and switch back to fragment where change is not reflected.
• Same as above in non-diff full suite editor.
• Modify in fragment diff editor. Save. Modify again and save. Prompt: "The content of the file is newer. Please compare your version with the file contents or overwrite the content of the file with your changes."

Also, adjust fs stat result to avoid re-opening already-opened full suite editor (see requestFs).