Tweak Karma CI config
Tweak Karma CI config

Looks like the previous updates still have some sporadic failures. This lowers the concurrency even more and increases the retry limit in an attempt to reduce flaky test failures.

segmentio/analytics.js-core,nice-fungal/analytics.js-core,nice-fungal/analytics.js-core,segmentio/analytics.js-core,segmentio/analytics.js-core