Not sure why unit tests failing frequently.
It may me be related to responsiveness of network connections, maybe a bug.

Related:

• Karma unit tests not running stable readable-web-to-node-stream#33
• Unit tests not running stable music-metadata-browser#46

Possibly the same issue:

• As of v1.5.0 karma-browserstack-launcher often times out karma-runner/karma-browserstack-launcher#152