Fixes #1513: Backspace on middle of whitespace only line fails

[Chillee]

So I fixed issue #1513. This issue only occurs when Configuration.expandTab is turned on.
The main behavioral decision I made is that if you press backspace on a whitespace only line, it deletes both to the next "tab" as well as all the whitespace after it.

I'm unsure about a couple of things

1. I'd like to create tests, but from looking through the repo, I don't see how to set Configuration.expandTab to be true while running these tests.

2. When running the tests locally, I get 56 tests failing, with or without my change. I don't really know what I'm supposed to do there (I thought I would try running the tests on CI, and cool! They passed.).

Cheers,
Horace

[xconverge]

Hmm yea we are supposed to be ignoring remappings etc when running tests but maybe that is not working so some of your remappings are impacting the tests.

This would be tough to write a test for unless you set the parameter manually and use the old style tests.

Like this one https://github.com/VSCodeVim/Vim/blob/master/test/mode/modeInsert.test.ts#L118

only set Configuration.expandtab = true as the first thing the test does

[Chillee]

I can't figure out how to set the configuration setting, but it seems like it's already the necessary setting by default, right?

Vim/test/testUtils.ts

Line 86 in 84d209f

Configuration.expandtab = insertSpaces;

[xconverge]

ah yea looks like it is set for this case already you are right

[Chillee]

I have added two new tests, both of which test the new behavior. However, I was running into some issues, and I believe they are related to issues with the test runner.

If you check the new continuous integration, there are 2 new tests, one of which is the other test but with an extra (non-blank) line. The first test works as expected, but the second test fails. I am unable to reproduce the second test's behavior in a non-test environment, and when viewing the running of the test, it seems to display some erratic behavior with regards to the cursor. I suspect this is a bug with the test runner.