For anyone still having this issue, doing this solved it for me.
Add this to your test target build settings.
HEADER_SEARCH_PATHS = "${SRCROOT}/Pods/Firebase/CoreOnly/Sources"
Full discussion can be found on firebase GitHub issues.
you can find HEADER_SEARCH_PATHS in Test Target > Build Settings > Search Paths > Header Search paths.
Just incase you cant find HEADER_SEARCH_PATHS in Build Settings, click the ALL button to show all settings.