fix: Make "Expand all" button more screen-reader friendly

[DaveTryon]

Description of changes

Issue #3539 suggests an improvement to the Expand all/Collapse all button when read via a screen reader. Specifically, set the aria-label property to a more descriptive value when the button is in the collapsed state. The issue says nothing about modifying the aria-label property in the expanded state. Change has been validated with NVDA in both Web and Unified clients.

HTML of the button in the collapsed state (aria-label property is new)

<button type="button" aria-expanded="false" aria-label="Expand all rules to show failed instances." class="ms-Button ms-Button--action ms-Button--command expand-collapse-all-button--3fwBF root-113" data-is-focusable="true"><span class="ms-Button-flexContainer flexContainer-108" data-automationid="splitbuttonprimary"><i data-icon-name="ChevronRight" role="presentation" aria-hidden="true" class="ms-Icon root-37 css-60 ms-Button-icon icon-114" style="font-family: FabricMDL2Icons;"></i><span class="ms-Button-textContainer textContainer-109"><span class="ms-Button-label label-57" id="id__18">Expand all</span></span></span></button>

HTML of the button in the expanded state (no aria-label is added, and button is unchanged from before):

<button type="button" aria-expanded="true" class="ms-Button ms-Button--action ms-Button--command expand-collapse-all-button--3fwBF root-113" data-is-focusable="true"><span class="ms-Button-flexContainer flexContainer-108" data-automationid="splitbuttonprimary"><i data-icon-name="ChevronDown" role="presentation" aria-hidden="true" class="ms-Icon root-37 css-60 ms-Button-icon icon-114" style="font-family: FabricMDL2Icons;"></i><span class="ms-Button-textContainer textContainer-109"><span class="ms-Button-label label-57" id="id__18">Collapse all</span></span></span></button>

Pull request checklist

• Addresses an existing issue: Expand/ Collapse All button could be more descriptive #3539
• Ran yarn fastpass
• Added/updated relevant unit test(s) (and ran yarn test)
• Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• PR title AND final merge commit title both start with a semantic tag (fix:, chore:, feat(feature-name):, refactor:). See CONTRIBUTING.md.
• (UI changes only) Added screenshots/GIFs to description above
• (UI changes only) Verified usability with NVDA/JAWS

[DaveTryon]

@ferBonnin, can you please clarify whether or not the aria-label is supposed to be modified when the button is expanded? If so, what should the intended aria-label text be?

[JGibson2019]

LGTM but waiting for confirmation from Fer about aria-label for expanded state

[ferBonnin]

@ferBonnin, can you please clarify whether or not the aria-label is supposed to be modified when the button is expanded? If so, what should the intended aria-label text be?

No changes required in the label for when the button is expanded. We can keep the same behaviour we have currently.