Expand tests for DruxtClient instantiation in packages/router/src/nuxt/index.js

[coderabbitai[bot]]

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As per this comment, we need to expand on the tests for DruxtClient instantiation in packages/router/src/nuxt/index.js. The code has been moved from a different file, but it would be beneficial to improve test coverage.\n\nOriginal PR: https://github.com/druxt/druxt.js/pull/697\nRequested by: @Decipher