Update Gutenberg Test Data to have more test block instances

[iamtakashi]

It'd be good to update Gutenberg test data. It's been 8 month since the last update, and since then some blocks have been updated such as core/cover that now takes more blocks inside.

It might be best to have a post for each block rather than a post for a group. Each post should have instances of the block with different configurations such as alignment and sizes of inner blocks. For example, Cover with all the headings, with paragraphs with different font sizes and alignments etc.