[ci] fix test.yaml workflow

[armcknight]

This gets the test.yml workflow running and successfully uploading the coverate report to codecov. I'll leave it to subsequent PRs to tweak the codecov config to your liking.

• get the tests running in test.yml; add make test
• remove upload-codecov-report.yml as the upload is now done in test.yml; add .codecov.yml config file as a starting point
• use slather to process the xcodebuild coverage report
• update macos image in GHA and the iphone destination; fix IPHONEOS_DEPLOYMENT_TARGET to use the xcode default latest version
• to avoid uploading build symbols from the test runs, add a new build config to the Xcode project named Test

[philipphofmann]

LGTM