Run tests using the standalone build in an empty context (#4668)

Validate that our build is stripping all the "native" dependencies (native modules and env vars).

scripts/build/shims/assert.js

@@ -1,4 +1,2 @@
-module.exports = {
-  ok() {},
-  strictEqual() {}
-};
+export function ok() {}
+export function strictEqual() {}

scripts/build/shims/os.js

@@ -0,0 +1 @@
+export const EOL = "\n";