Steps to reproduce (assumes ChromeVox or similar)

1. Open https://elastic.github.io/eui/#/display/tables in a new tab.
2. Tab to the table under the "Adding selection to a BasicTable" heading.
3. Tab to the the first "select all" checkbox in the table

Actual Result

"Checkbox Not Checked"

Expected Result

"Select All Rows. Checkbox not Checked"

Category: #688: Elastic UI Tables Accessibility
Relevant WCAG Criteria: 3.3.2 Labels or Instructions