Normalize whitespace in getAccessibilityTree element accessible names

[calebeby]

Closes #471

From the changeset:

Normalize whitespace in element accessible names in getAccessibilityTree. Markup with elements which have an accessible name that includes irregular whitespace, like non-breaking spaces, will now have a different output for getAccessibilityTree snapshots. Previously, the whitespace was included, now, whitespace is replaced with a single space.