Fix TabSheetElement clicks for IE, test tweaks. (#12291)

Fix TabSheetElement clicks for IE, test tweaks. (#12291)

- Backspace navigation hasn't worked in years thanks to new browser
standards, removed related tests. Left a test for regular backspace use
within a modal window.
- Enabled a modal window test for Chrome since it seems to be working
now, removed extending of another test class to avoid running the tests
twice without any configuration changes.
- Adjusted browser width limitation.
- Added screenshots.