code coverage with babel istanbul (#4649)

* WIP add instanbul code coverage to vue files

* move webpack coverage config to a separate file for test:coverage, pin dependencies, remove istanbul-instrumenter-loader

* dont include node_modules in babel config, it breaks istanbul for some reason

* ignore spec files from coverage

* document coverage files, remove unused karma config

* use test instead of test:coverage in config.yml, disable code and link to issue to enable coverage in Vue <template>s

webpack.coverage.js

@@ -0,0 +1,42 @@
+// This file extends the webpack.dev.js config to add istanbul coverage
+// instrumentation using babel-plugin-istanbul (see babel.coverage.js)
+
+const config = require('./webpack.dev');
+
+const path = require('path');
+
+config.devtool = false;
+
+const vueLoaderRule = config.module.rules.find(r => r.use === 'vue-loader');
+
+vueLoaderRule.use = {
+    loader: 'vue-loader'
+    // Attempt to use Babel with babel-plugin-istanbul
+
+    // TODO The purpose of this was to try to add coverage to JS expressions
+    // inside `<template>` markup, but it seems to add only coverage inside
+    // `<script>` tags.
+    // Issue: https://github.com/nasa/openmct/issues/4973
+    //
+    // options: {
+    //     compiler: require('vue-template-babel-compiler'),
+    //     compilerOptions: {
+    //         babelOptions: require('./babel.coverage')
+    //     }
+    // }
+};
+
+config.module.rules.push({
+    test: /\.js$/,
+    // test: /(\.js$)|(\?vue&type=template)/,
+    // exclude: /node_modules(?!.*\.vue)/,
+    exclude: /(Spec\.js$)|(node_modules)/,
+    use: {
+        loader: 'babel-loader',
+        options: {
+            configFile: path.resolve(process.cwd(), 'babel.coverage.js')
+        }
+    }
+});
+
+module.exports = config;