refactor: Use ES6, Test with AVA

- Use async/await instead of callbacks
- Use execa to run command line
- Use AVA for tests
- Add several assertions in the unit tests
- Add documentation (comments) in the tests
- Run tests with a real git repo instead of mocking child_process and add test helpers to create repos, commits and checkout
- Simplify test directory structure
- Simplify code readability (mostly with async/await)
- Use eslint for for linting, prettier for formatting

src/lib/plugins.js

@@ -1,42 +1,42 @@
-var relative = require('require-relative')
-var series = require('run-series')
+const {promisify} = require('util');
+const relative = require('require-relative');
+const pSeries = require('p-series');
 
-var exports = module.exports = function (options) {
-  var plugins = {
-    analyzeCommits: exports.normalize(options.analyzeCommits, '@semantic-release/commit-analyzer'),
-    generateNotes: exports.normalize(options.generateNotes, '@semantic-release/release-notes-generator'),
-    getLastRelease: exports.normalize(options.getLastRelease, '@semantic-release/last-release-npm')
-  }
-
-  ;['verifyConditions', 'verifyRelease'].forEach(function (plugin) {
+module.exports = options => {
+  const plugins = {
+    analyzeCommits: normalize(options.analyzeCommits, '@semantic-release/commit-analyzer'),
+    generateNotes: normalize(options.generateNotes, '@semantic-release/release-notes-generator'),
+    getLastRelease: normalize(options.getLastRelease, '@semantic-release/last-release-npm'),
+  };
+  ['verifyConditions', 'verifyRelease'].forEach(plugin => {
     if (!Array.isArray(options[plugin])) {
-      plugins[plugin] = exports.normalize(
+      plugins[plugin] = normalize(
         options[plugin],
-        plugin === 'verifyConditions'
-          ? '@semantic-release/condition-travis'
-          : './plugin-noop'
-      )
-      return
+        plugin === 'verifyConditions' ? '@semantic-release/condition-travis' : './plugin-noop'
+      );
+      return;
     }
 
-    plugins[plugin] = function (pluginOptions, cb) {
-      var tasks = options[plugin].map(function (step) {
-        return exports.normalize(step, './plugin-noop').bind(null, pluginOptions)
-      })
+    plugins[plugin] = async pluginOptions => {
+      return pSeries(
+        options[plugin].map(step => {
+          return () => promisify(normalize(step, './plugin-noop'))(pluginOptions);
+        })
+      );
+    };
+  });
 
-      series(tasks, cb)
-    }
-  })
+  return plugins;
+};
 
-  return plugins
-}
+const normalize = (pluginConfig, fallback) => {
+  if (typeof pluginConfig === 'string') return relative(pluginConfig).bind(null, {});
 
-exports.normalize = function (pluginConfig, fallback) {
-  if (typeof pluginConfig === 'string') return relative(pluginConfig).bind(null, {})
-
-  if (pluginConfig && (typeof pluginConfig.path === 'string')) {
-    return relative(pluginConfig.path).bind(null, pluginConfig)
+  if (pluginConfig && typeof pluginConfig.path === 'string') {
+    return relative(pluginConfig.path).bind(null, pluginConfig);
   }
 
-  return require(fallback).bind(null, pluginConfig)
-}
+  return require(fallback).bind(null, pluginConfig);
+};
+
+module.exports.normalize = normalize;