Plan build-only verification for packages with no test targets

[coenttb]

Problem

Full-tier CI invokes the package test operation for packages that declare no test targets. SwiftPM then exits with error: no tests found, producing identical false-red build legs across platforms even though the required build and quality checks are green.

Exact-head evidence:

• Witness Observation: https://github.com/swift-foundations/swift-witness-observation/actions/runs/31951524460
• Witness Test Support: https://github.com/swift-foundations/swift-witness-test-support/actions/runs/31951524471
• Witness Context: https://github.com/swift-foundations/swift-witness-context/actions/runs/31951524910

Required outcome

• Derive test-target presence from the package graph during planning.
• Packages with test targets continue to run the test operation.
• Packages without test targets receive a build-only verification operation and do not fail merely because SwiftPM reports no tests.
• Keep the decision typed and central; package callers remain input-free and packages do not choose their CI behavior.
• Add positive and negative fixtures proving both planner dispositions.
• Fresh full-tier runs for the three exact package heads complete green without package-local configuration.