do not recursively call process.exit() (#3684)

* do not recursively call `process.exit()`

When inside `process.on('exit')`, calling `process.exit()`
stops other `exit` event listeners from being invoked.

This has lead to issues with coverage generation with `nyc`
because a fallback exit detection mechanism in it that relies
on internal Node.js APIs was accidentally disabled in Node v11.7.0,
leaving `nyc` with no way to detect process exit.

Refs: nodejs/node#25650

* Revert "pin default Node.js version to v11.6.0 in .travis.yml"

This reverts commit 1abfef6.

.travis.yml

@@ -11,7 +11,7 @@ stages:
 
 # defaults
 language: node_js
-node_js: '11.6'
+node_js: '11'
 addons:
   apt:
     packages:

lib/cli/run-helpers.js

@@ -25,7 +25,7 @@ const cwd = (exports.cwd = process.cwd());
  */
 const exitMochaLater = code => {
   process.on('exit', () => {
-    process.exit(Math.min(code, 255));
+    process.exitCode = Math.min(code, 255);
   });
 };