Issue #3631203: Don't clone the content blocks of pattern previews

Closes #3631203

The duplicate check of content blocks (editor.BlockEdit filter in js/filters/content-blocks.jsx) skips previews, so previewing a pattern in the inserter no longer clones the content blocks it shows. It reads the preview mode under both names of the setting: __unstableIsPreviewMode (the WordPress 6.4 packages of 3.0.x) and isPreviewMode (WordPress 6.8+). Inserting a pattern still checks its content blocks and clones the used ones. The filter's console.log() calls are removed.

ContentBlockPatternPreviewTest previews a pattern whose content block is used by another node (no content block is created), then inserts it (the block is cloned). closeWelcomeGuide() moved to the base class, as the test logs in a second user.