[macOS] API tests ChangeAttachmentDataUpdatesWithInPlaceDisplay and InPlaceVideoAttac...

[macOS] API tests ChangeAttachmentDataUpdatesWithInPlaceDisplay and InPlaceVideoAttachmentInsertionWithinList are failing
https://bugs.webkit.org/show_bug.cgi?id=181626

Reviewed by Joseph Pecoraro.

Make the web view for WKAttachmentTests slightly bigger to avoid size clamping behavior introduced in r226915.
Additionally, write a new API test covering the change in r226915.

* TestWebKitAPI/Tests/WebKitCocoa/WKAttachmentTests.mm:
(webViewForTestingAttachments):
(TestWebKitAPI::TEST):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@226935 268f45cc-cd09-0410-ab3c-d52691b4dbfc