Add WorkoutsTests unit test target

No unit test target existed (testTargets: [] in scheme). Adds a
bundle.unit-test target wired into the Workouts scheme, with a smoke test
that round-trips SplitDocument through IndieSync's DocumentCoder — the
same codec DocumentFileStore uses for every iCloud document.

project.yml

@@ -72,7 +72,23 @@ targets:
         TARGETED_DEVICE_FAMILY: "1"
         DEVELOPMENT_ASSET_PATHS: "\"Workouts/Preview Content\""
     scheme:
-      testTargets: []
+      testTargets:
+        - WorkoutsTests
+
+  # ---- unit tests (domain model / codec, against the iOS app target) --------
+  WorkoutsTests:
+    type: bundle.unit-test
+    platform: iOS
+    sources:
+      - WorkoutsTests
+    dependencies:
+      - target: Workouts
+      - package: IndieSync
+    settings:
+      base:
+        PRODUCT_BUNDLE_IDENTIFIER: dev.rzen.indie.WorkoutsTests
+        GENERATE_INFOPLIST_FILE: true
+        SWIFT_STRICT_CONCURRENCY: complete
 
   # ---- watchOS app (no iCloud; syncs through the phone via WatchConnectivity) -
   Workouts Watch App: