[sitecore-jss-react] [ErrorBoundaries] Added unit tests to cover withPlaceholder HOC

[illiakovalenko]

• This PR follows the Contribution Guide
• Changelog updated
• Upgrade guide entry added

Description / Motivation

• Introduced unit tests for withPlaceholder HOC to cover ErrorBoundaries functionality
• Removed extra/invalid shapes of data (with/without params data). Since params property doesn't affect Placeholder functionality, unless it has FieldNames)

Testing Details

• Unit Test Added
• Manual Test/Other (Please elaborate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[yavorsk]

Looks good to me.
There is only one thing - not connected with this PR but wanted to mention - EditMode import seems to be not needed in withPlaceholder.test.tsx