fix(build): make sure coverage plugin is only loaded in addon tests

[anehx]

https://github.com/kategengler/ember-cli-code-coverage/issues/347#issuecomment-1488101559

[anehx]

:tada: This PR is included in version 7.0.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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