Bug fixes for Ctrl+T in insert mode.

Here are two fixes for bugs in the insert-mode flavor of shifting the line right.

1. In Vim `Ctrl+T` in insert mode on a empty line indents by the shiftwidth setting.  In VsVim, this has no effect because the line is blank.
2. In Vim `Ctrl+T` in insert mode when there is non-blank text before the caret rounds the leading blanks up to a multiple of the shiftwidth.  In VsVim, it shifts the line by exactly shiftwidth spaces.

By comparison, `Ctrl+D` was already implemented in VsVim as Vim would do it.  These changes make `Ctrl+T` symmetrical with `Ctrl+D` and compatible with Vim.

I added two unit tests for these two cases.

Also included is a commit to fix two failing unit tests due to incomplete mocking setup.