Travis failing with Kitura-NIO on Mac

[djones6]

Context and Description

Around the beginning of February, Travis started to fail when running with KITURA_NIO on Mac.

• Last successful run against master: https://travis-ci.org/IBM-Swift/Kitura/jobs/640258160
• First failing build: https://travis-ci.org/IBM-Swift/Kitura/jobs/640728526

Failure appears to be async waits timing out (presumably, no response received) intermittently. From skimming over the test output, it looks like the timeouts occur in random tests but always when running with SSL enabled. Maybe something has changed in swift-nio or swift-nio-ssl that has exposed a problem?

The failure is very consistent running under Travis, but doesn't seem to be easy to replicate locally - running env KITURA_NIO=1 swift test --parallel locally on a Mac, all the tests pass.

I tried updating the Travis to Swift 5.1.3 and Xcode 11.3, but that didn't help (https://travis-ci.org/IBM-Swift/Kitura/jobs/657704066). Turning off parallel testing didn't help either (https://travis-ci.org/IBM-Swift/Kitura/jobs/657753725)

This is causing failures in Travis for PRs raised since then, eg: #1491