`placeholder` attribute test

Pass Criteria

#1: Attribute is supported
#2: Correct HTML and Core Accessibility API mappings

Reference

Test ph-001: `placeholder`

placeholder should be used in accessibility mapping.

Test ph-002: `placeholder` with `aria-placeholder`

placeholder should be used in accessibility mapping.

Test ph-003: Empty `placeholder` with `aria-placeholder`

aria-placeholder should be used in accessibility mapping.