ci: use --coverage option when running `embark test` in test dapps

[michaelsbradleyjr]

Avoid a situation where the --coverage option is not working (to an extent that embark test --coverage can't run successfully) but the problem goes undetected prior to the next stable release.

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This PR is in draft because on master currently, and going back to at least the stable release of 5.0.0, embark test --coverage in dapps/tests/app doesn't work correctly (there's an unhandled exception) and it hangs indefinitely. Once that bug has been fixed, this PR can be taken out of draft and merged.

[ghost]

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[iurimatias]

as note, the tests also seem to time out

[michaelsbradleyjr]

as note, the tests also seem to time out

Yes, that behavior is related to the bug.

We'd only merge this PR after we fix the bug, the point being to catch in CI a regression or future bug/s re: --coverarge that are impactful enough to cause embark test --coverage to not complete successfully.