Exclude code coverage for cross product changes

[granluo]

#8295
RTDB was changed in this PR so incremental code coverage is running, but this change is in FirebaseCore and we wouldn't expect full code coverage on FirebaseCore from the RTDB tests.
#8295 (comment)_

[granluo]

The tool will actually search files in the xcresult bundle. And FirebaseDatabase xcresult bundles contains files under firebase-ios-sdk/FirebaseCore/Sources/, that why the FirebaseCore/Sources/FIRApp.m is caught. The xcresult bundle contains files from the following dirs:

firebase-ios-sdk/Firebase/CoreDiagnostics/FIRCDLibrary/FIRCoreDiagnostics.m
firebase-ios-sdk/FirebaseCore/Sources/*
firebase-ios-sdk/FirebaseDatabase/Sources/*
firebase-ios-sdk/FirebaseDatabase/Tests/*
firebase-ios-sdk/SharedTestUtilities/*

 /var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/CocoaPods-Lint-20210625-2536-4gmwka-FirebaseDatabase/Pods/*

We probably just need to involve firebase-ios-sdk/FirebaseCore/Sources/*

[granluo]

files-for-target seems to be a way to just output coverage for a specific target.
For files, we might need to parse the outcome in postprocess, like filter files by dir/path.

[paulb777]

Closing for staleness. Please open a new issue and refer to this one if it is still occurring in recent releases.