Add create-project test coverage of development configs

[edmorley]

As found in #852, create-project currently doesn't have any test coverage of the development (ie yarn start) configs. Whilst they are unit-tested in the individual presets, this can still miss issues:
#852 (comment)

Testing the dev server means dealing with running the server, then killing it after N seconds, and detecting the difference between startup errors and the process being killed by the test runner.

A simpler alternative that might help in the meantime, would be to test a webpack build using --mode development, to give at least some of the coverage.