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/verify.js

@@ -1,40 +1,25 @@
-var SemanticReleaseError = require('@semantic-release/error')
+const SemanticReleaseError = require('@semantic-release/error');
 
-module.exports = function (config) {
-  var pkg = config.pkg
-  var options = config.options
-  var env = config.env
-  var errors = []
+module.exports = ({pkg, options, env}) => {
+  const errors = [];
 
   if (!pkg.name) {
-    errors.push(new SemanticReleaseError(
-      'No "name" found in package.json.',
-      'ENOPKGNAME'
-    ))
+    errors.push(new SemanticReleaseError('No "name" found in package.json.', 'ENOPKGNAME'));
   }
 
   if (!pkg.repository || !pkg.repository.url) {
-    errors.push(new SemanticReleaseError(
-      'No "repository" found in package.json.',
-      'ENOPKGREPO'
-    ))
+    errors.push(new SemanticReleaseError('No "repository" found in package.json.', 'ENOPKGREPO'));
   }
 
-  if (options.debug) return errors
+  if (!options.debug) {
+    if (!options.githubToken) {
+      errors.push(new SemanticReleaseError('No github token specified.', 'ENOGHTOKEN'));
+    }
 
-  if (!options.githubToken) {
-    errors.push(new SemanticReleaseError(
-      'No github token specified.',
-      'ENOGHTOKEN'
-    ))
+    if (!(env.NPM_TOKEN || (env.NPM_OLD_TOKEN && env.NPM_EMAIL))) {
+      errors.push(new SemanticReleaseError('No npm token specified.', 'ENONPMTOKEN'));
+    }
   }
 
-  if (!(env.NPM_TOKEN || (env.NPM_OLD_TOKEN && env.NPM_EMAIL))) {
-    errors.push(new SemanticReleaseError(
-      'No npm token specified.',
-      'ENONPMTOKEN'
-    ))
-  }
-
-  return errors
-}
+  return errors;
+};