Suite shard plans come from server-side history plus a catch-all shard

Suite-granularity shard plans are now produced completely on the server side from the .xctestrun module list plus historical TestSuiteRun data, instead of letting the CLI boot every test bundle with xcodebuild -enumerate-tests. This eliminates the flakiness and runtime cost of simulator-bundle booting under contention; unknown or newly added suites are caught by a final catch-all shard that uses -skip-testing to run everything not explicitly assigned.