chore: make build more efficient by running tests as .js

[rix0rrr]

We are now running every test file through ts-jest, which means all TypeScript
code gets parsed again and again for every test.

Instead, in this PR we run our tests in 2 different ways:

• In a dev environment, as TypeScript for quick iteration
• In a CI environment, as JavaScript for higher throughput execution

Needs some related changes to the tests to make it all work out properly:

• We need to update the snapshot resolver to always name the snapshots *.ts.snap (not sometimes *.ts.snap and sometimes *.js.snap)
• We need to explicitly set up babel-jest so that mock hoisting still works.
• We must use the jest.mock() implicit global in order for mock hoisting to work with babel-jest, so can't have explicit imports.
• cx-api was not compiling its tests so we couldn't run .js tests.
• Had to remove a stray custom resource test, as it was not being run and was in a broken state that took too much effort to fix.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license