Only run tests from the tests/unit directory

[rlindskog]

Currently, vue-cli-plugin-unit-karma is running all tests in the tests directory. This is a problem, because it's also attempting to run my vue-cli-plugin-e2e-cypress tests in the tests/e2e directory.

According to the docs, it should only be running tests/unit.
https://github.com/davidwallacejackson/vue-cli-plugin-unit-karma#usage

This PR changes it to only run tests in the tests/unit directory

[smoothdvd]

@rlindskog You can use

pluginOptions: {
    karma: {
      files: [
        'tests/unit/**/*.spec.js',
      ]
   }
}

override this option in vue.config.js