Make DX improvements for writing tests for new dApp e2e

Make DX improvements for writing tests for new dApp API e2e tests.

For interacting with the provider only.

Related to 4 test files from [this PR](https://github.com/MetaMask/metamask-extension/pull/18213) and [this ticket](https://app.zenhub.com/workspaces/extension-client-team-63529dce65cbb100265a3842/issues/gh/metamask/metamask-extension/19341).

**Acceptance Criteria**
- Extract common testing patterns into new methods so they are reusable across the folder
- Documentation
- Add Readme "how to test dApp API method (reference existing implementation)

Start by reviewing the four dApp API e2e test files from PR 18213 and the related issue 19341. Identify repeated provider-interaction patterns, then check the folder's README or add one describing how to test a dApp API method by referencing an existing implementation. Done means reusable methods exist, the documentation is present, and the affected e2e tests still pass.

Written by the indexing model from the issue text.