[Bug] Doesn't work when Storybook is built with the new --test flag

[otaviomad]

Describe the bug

Storybook 7.6 introduced a new --test flag for building Storybook with less of the fluff that is not needed for testing environments.
When you try to run test-storybook with --coverage on a storybook that was built with --test, I get the following error:

The code in this story is not instrumented, which means the coverage setup is likely not correct.

Steps to reproduce the behavior

1. Build a Storybook with --test
2. Run test-storybook on that with --coverage
3. Rebuild the same Storybook without --test
4. Test storybook passes

Expected behavior

I believe that the istanbul plugin is not being properly injected when --test is enabled. I don't get the [addon-coverage] Adding istanbul plugin to Vite config message when I use it. The coverage should work when --test is enabled.

[vicrep]

Experiencing the same issue here.

Did a little digging and it seems that the coverage add-on is disabled by default if not overridden? https://github.com/storybookjs/storybook/blob/00124ed94f397d2d6c0a9fa8bbad823cfa1b0ab9/code/lib/core-server/src/presets/common-override-preset.ts#L41

[vicrep]

I can confirm that if you are using @storybook/test-runner, that the issue is indeed just that -- that the coverage addon gets removed by default when running a build with the --test flag enabled.

This should probably be fixed, but in the meantime, overriding your build configuration as such does mitigate the issue:

// main.ts
{ ...,
  build: {
    test: {
      disabledAddons: [
        '@storybook/addon-docs',
        '@storybook/addon-essentials/docs',
      ],
    },
  },
}

To maintainers: This does seem like it was accidentally added to the block-list, I'm happy to propose a PR in the SB repo to change that if that is indeed something that shouldn't be set by default -- feels a little odd to me to disable coverage when making a test build of your storybook, which is the prime use-case for coverage.

[jonniebigodes]

@otaviomad, thanks for reporting this, and also @vicrep appreciate your assistance on this 🙏 . So that you're aware, I've pushed a pull request here and will push another into the official documentation to address this until we have a proper fix.

Hope you all have a great day.

Stay safe