Move test helpers to core

[alschmiedt]

Is your feature request related to a problem? Please describe.

We use test helpers found in dev tools to write tests in core. The fact that we were using an external package was causing pain while trying to do the goog_module conversion because we kept having to release dev-tools to get CI to pass.

Describe the solution you'd like

• We need to move test helpers out of dev-tools and into core.
• Test that we can successfully import them from the blockly package.

Describe alternatives you've considered

Additional context