Fix indentation spec

Calling insert multiple times will reset indentation, we have to make
sure we enter text with a single insert statement to test indentation.

Now it fails (as expected)
First actual passing spec

Unsure why I have to use prev_indent + 5 to end up with 4 spaces.

Ideally, I should be able to call prev_indent + &shiftwidth, but that
results in something even worse.