Test: Support tab completion for snippets

[Tyriar]

Test for #9579:

• Windows - @egamma
• OS X - @joaomoreno
• Linux - @aeschli

There is a new configuration property editor.tabCompletion which defaults to false. When set to true verify that

• pressing the Tab key when the cursor follows a snippet prefix expands the snippet
• reason about the name of this setting (tabCompletion) - rationale was that folks know this feature as tab completion
• make sure things are live: run the Preferences: Snippet to define a new snippet and make sure it works without restart
• use prefixes with non-word characters like c'l and make sure it works

[gerane]

@Tyriar Would this work for extension snippets as well? Meaning if you have an extension that creates a new snippet, would it still require a restart for the new extension snippet to be loaded?

[jrieken]

yes - in order to activate extension you need to reload the editor

[aeschli]

I like it, I think we should enable it by default.
The conflict with Emmet is not really a problem given that we have no default snippets for html and the user just needs to pick non conflicting snippet names.

[jrieken]

@aeschli Ideas about the name tabCompletion vs snippetTabCompletion?

[joaomoreno]

I like the current proposal, but I think adding the prefix snippet would benefit it as the feature only revolves around snippets...

[aeschli]

I'd stick with tabCompletion, it seems to be an established term e.g. in shells. snippetTabCompletion is a strange construct, maybe rather snippetCompletionOnTab or snippetExpansionOnTab