check-tests.js not generated before watch/serve mode starts

[chrispcampbell]

There is a timing issue in the way plugin-check runs in watch/serve mode. If a testConfigPath is not provided, we generate a default config, but currently this step is skipped in postBuild in development mode, since it supposed to be covered by watch. But for an initial sde watch, there is a race condition, so the check-tests.js may not be created before the report dev server starts.

To resolve this, we should probably just run the genTestConfig step in development mode, only for the initial postBuild call.