Test rewriting support for JSON configuration files

[weinand]

Test for #xxx:

• Windows - @alexandrudima
• OS X - @jrieken
• Linux - @dbaeumer
• open an editor.
• open the mode selection dialog (lower right status bar) and select 'Configure File Association to *.xy'.
• this will open the user settings and will add a new key "files.associations": { "*.xy": languageId }

Verify that:

• This works with 'files.associations' is already existing or not, has entries or not.
• This works with an empty settings file & a blank settings file
• That the added content is properly formatted, but that existing content is kept as is
• Test how it behaves with corrupt content, unsaved setting....