🐛 Flush stdout before exiting process after running tests

[rsimha]

When we exit Karma after running tests, it sometimes prematurely exits while its reporters are busy writing to the console. This also breaks Travis log folding, and makes the logs difficult to read. For example, see https://travis-ci.org/ampproject/amphtml/jobs/367291014#L1391-L1397

Unfortunately, this is due to the asynchronous nature of stdout in nodejs. See nodejs/node#6456, nodejs/node#6379, yarnpkg/yarn#5005, etc.

This PR inserts process.stdout.write('\n') before each call to process.exit, since that will cause the output to get flushed.

[rsimha]

/to @jridgewell @erwinmombay @danielrozenberg

[rsimha]

Flushing the stdout before exiting didn't work. #14669 contains a new approach that doesn't rely on process.exit to quit the Karma server.