Consider negative tabindex a null tabindex

[kevee]

Saucelabs test: https://saucelabs.com/tests/f14a244f838a4687b9e19668c6119ae6

If you have an empty tabindex attribute, IE (11-8) will change it to a negative number. Negative numbers are allowed in the spec, but as our current tabindex tests are concerning form elements, a negative number should be considered a failure. This affects input*HasTabIndex tests.