Build ESM -> CJS before running tests in browsers via Saucelabs

Noteworthy tweak is the fact that it seems `zuul` are eagerly loading
all `require('whatever-package-or-path')` it sees before pushing the
contents to Saucelabs.

That was troublesome because we don't want the `esm` package to be
loaded when running browser tests. But it was, even though we loaded
`esm` conditionally after checking the current Node.js version used.

The trick was to not use `require('esm')` but `module.require('esm')`
instead, to fool the assumed look-out for `require` somewhere inside
`zuul`'s code.

.travis.yml

@@ -2,7 +2,7 @@ language: node_js
 node_js:
   - 8
 script:
-  - npm test
+  - npm run build
   - "test $TRAVIS_PULL_REQUEST != 'false' || test $TRAVIS_NODE_VERSION != '8' || npm run test-browser"
 env:
   global:

test/helper.js

@@ -1,12 +1,18 @@
 var chai = require('chai');
-var nodejsMajorVersion = Number(process.versions.node.split(".")[0]);
+var isRunningInNode = process !== undefined && process.versions.node !== undefined;
 
-isLegacyNodeVersion = !(nodejsMajorVersion >= 10);
+if (isRunningInNode) {
+  var nodejsMajorVersion = Number(process.versions.node.split('.')[0]);
+  isLegacyNodeVersion = !(nodejsMajorVersion >= 10);
 
-if (!isLegacyNodeVersion) {
-  require = require("esm")(module);
+  if (!isLegacyNodeVersion) {
+    // The `zuul` package we use to run tests in browsers via Saucelabs eagerly loads all
+    // packages it sees being used via `require()`. Because we don't want the `esm` package
+    // to be loaded when running browser tests, we refer to `require()` via `module.require()`
+    // because that avoid the mentioned eager loading
+    module.require = module.require('esm')(module);
+  }
 }
-
 assert = chai.assert;
 chai.should();
 Mustache = require('../mustache');