Use captureWarning helper from dev-tools

`captureWarnings` is now available available on on testHelpers from dev-tools after https://github.com/google/blockly-samples/issues/567.

The `captureWarnings` definition in `mocha/tests/test_helpers.js` should be removed and the usages of `captureWarnings` in the tests under `mocha/tests/` should use the `captureWarnings` function defined on `testHelpers` from dev-tools.