Add karma for testing (#2)

karma.conf.cjs

@@ -0,0 +1,24 @@
+const { createDefaultConfig } = require('@open-wc/testing-karma');
+const merge = require('deepmerge');
+
+module.exports = config => {
+  config.set(
+    merge(createDefaultConfig(config), {
+      frameworks: ['mocha', 'chai'],
+      client: {
+        mocha: {ui: 'tdd'}
+      },
+      files: [
+        {
+          pattern: config.grep ? config.grep : 'test/**/*_test.js',
+          type: 'module'
+        },
+      ],
+      // See the karma-esm docs for all options
+      esm: {
+        nodeResolve: true,
+      },
+    }),
+  );
+  return config;
+};