I\'ve been banging my head against a wall with this for the last few days but despite multiple Google/SO/Github searches I can\'t find a resolution to the issues I\'m having
The problem is recorded in the firebase project here:
https://github.com/firebase/firebase-ios-sdk/issues/58
There is a workaround:
Add "${PODS_ROOT}/Firebase/Core/Sources" to your Tests target only under Build Settings -> Header Search Paths
but this is also fixed by upgrading to CocoaPods 1.4.0 or later, which is a better solution.
At the time I'm writing this (November 2017) cocoapods 1.4.0 is still in beta, so to install it you need to explicitly request the beta:
gem install cocoapods --pre
This and then doing a pod install solved the problem running my tests.