fix test coverage

karma.files.js

@@ -0,0 +1,16 @@
+import Vue from "vue";
+
+Vue.config.productionTip = false;
+
+// require all test files (files that ends with .spec.js)
+const testsContext = require.context("./tests/unit", true, /\.spec.js$/);
+
+// Read more about why we need to call testContext:
+// https://www.npmjs.com/package/require-context#context-api
+testsContext.keys().forEach(testsContext);
+
+// require all src files except main.js for coverage.
+// you can also change this to match only the subset of files that
+// you want coverage for.
+const srcContext = require.context("./src", true, /^\.\/(?!main(\.js)?$)/);
+srcContext.keys().forEach(srcContext);