[npm] upgrade to mocha 5.2.0

[spalger]

Now that mocha has patched and released mochajs/mocha#3346 we can upgrade to 5.2.0 which was the reason we had to downgrade mocha in #19238.

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[epixa]

I created some test failures throughout the codebase to run with this change, and they were handled properly for intake and selenium. In the x-pack job, they did error, but the error message was swallowed and another useless message was shown instead.

My commit was afe91b1

Here's the selenium job that failed with that change on top of this PR: https://kibana-ci.elastic.co/job/elastic+kibana+pull-request+multijob-x-pack/1359/console

Here's the same tests run against master (to be specific, with this mocha bump reverted): https://kibana-ci.elastic.co/job/elastic+kibana+pull-request+multijob-x-pack/1365/console

As you can see, the tests failures without your change resulted in a nice and clear error message and stack trace at the bottom of the test output, as you'd expect: Error: expected true to equal false

However, when run with these changes, there is no clear error message and the error that appears seems to be something else entirely: test.titlePath is not a function

If you're interested, this is all going down in this temporary PR: #19268

[spalger]

Wow, @epixa want to just write tests for Mocha? :P

I'm not interested in fixing mocha, lets just stick with what we know works... Too bad