fix(tests): wait for the workspace to render before starting test actions

[rachel-fenichel]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Resolves

Fixes some tests that were flaky because the workspace wasn't always fully loaded.

Proposed Changes

In the testSetup function that loads a URL, wait for the workspace's block canvas to exist before returning.

Reason for Changes

The workspace must exist before the test actions start running. Checking for the block canvas is an easy test that uses standard webdriver APIs (waitForExist and selectors).