Introduce TestWebKitAPI mechanism for directly testing IPC serialization
<rdar://problem/117511848>
Pull request: https://github.com/WebKit/WebKit/pull/19579
Committed 269802@main (d9fd3e0d1f7a): <https://commits.webkit.org/269802@main> Reviewed commits have been landed. Closing PR #19579 and removing active labels.