Fix :tabe {file} only relative to current file (#1162)

[arussellk]

Fixes #1162

What this PR does / why we need it

This PR extends the logic for :tabe to better match user expectation. :tabe now handles the following situations:

1. :tabe {file} when you have a workspace open will try to open {file} relative to the root of your first workspace folder
2. :tabe {file} when you do not have a workspace will fall back to open {file} relative to your current document
3. :tabe will open an untitled document
4. :tabe {absolutePath} will open {absolutePath}

Which issue(s) this PR fixes

Fixes #1162

Special notes for your reviewer

1. I was unable to use fs.mkdir to make a folder because I kept getting a EACCES: permission denied, mkdir error. Because of that, I did not add tests for when a user is in a workspace. I did add tests for the other cases that :tabe handles.
2. I am now exporting testUtils.ts function createRandomFile for use in my tests.

[TravisBuddy]

Travis tests have failed

Hey Russell Kennington,
Please read the following log in order to understand the failure reason.
It'll be awesome if you fix what's wrong and commit the changes.

Node.js: 8

gulp forceprettier

[19:21:11] Using gulpfile ~/build/VSCodeVim/Vim/gulpfile.js
[19:21:11] Starting 'forceprettier'...
[19:21:23] Finished 'forceprettier' after 12 s

[jpoon]

Agreed that we should make this async. However, can you update the other return paths to also return a promise?

[arussellk]

@jpoon I awaited everything I could with the following two exceptions:

1. await vscode.commands.executeCommand('workbench.action.openEditorAtIndex1'); never resolves. I think it may be a bug with VSCode, so I should reproduce it in a small extension and open an issue on the VSCode repo.
2. vscode.commands.executeCommand('activeEditorMove') needs to be updated to vscode.commands.executeCommand('moveActiveEditor'). It isn't quite a simple name replacement, so it should probably be done in a different PR.

[jpoon]

Filed #2401 for the refactoring.

[jpoon]

Thanks @arussellk