Fix Buildkite Test Analytics

[crazytonyli]

Test Analytics stopped working after #321 was merged. The cause is, we are using xcodebuild (via fastlane) to run the Swift Package's test suites, and we have to add the Buildkite Test Analytics related environment variables to the scheme or testplan (see https://github.com/buildkite/test-collector-swift#step-3).

Here is the original Test Plan which is deleted in #321: https://github.com/wordpress-mobile/WordPress-iOS-Shared/blob/346953ec9fc99433c14b2c7c1298996a3fa70df0/WordPressSharedTests/TestPlan.xctestplan

[mokagio]

After the initial interest, I don't think we've used Test Analytics much. FWIW, I consider this a low priority fix.