Reusable Blocks: Move E2E tests setup logic to a beforeAll()

[noisysocks]

The tests in packages/e2e-tests/specs/editor/various/reusable-blocks.test.js have an assumed order which is incorrect as tests should be able to run independently of one another.

From #7947 (comment):

There should ideally be no concept of "above" as assuming tests to be run in particular order (or even that any others are run at all). I should be able to change this to it.only and the test should be fine to run in isolation. Common behaviors should be established in lifecycle (beforeEach or beforeAll) or repeated in every test case.

Note: I'm encountering this in implementing a new end-to-end test case for verifying the fix at #21205. Ideally I'd like to iterate on the test case with it.only, but at least as currently presented, there's no prior art for me to leverage commonly available reusable blocks to work with. For the purposes of my test, I might just copy the logic from "can be created" to create the reusable block. It could be straight-forward enough to refactor to its own function; not sure if I'll do that or end up just following the pattern here of reusing the block created in an earlier test case once finished.

[noisysocks]

As part of this refactor we can remove the 'can be inserted after refresh' test added in #20605 as it is redundant if the 'can be inserted' test begins from a clean slate.

[youknowriad]

I did an extensive refactor to this test a few weeks ago and I believe it's now based on independent tests.