[Tests] redo spawning test (w/o AssetProcessor)

[jhanca-robotecai]

What does this PR do?

SimulationInterfaces.Editor.Tests depended on ${PROJECT_NAME}.Assets just to get a processed .spawnable for SpawnAppTest. That target runs AssetProcessorBatch, which pulls the whole builder half of the engine into the CI build (EMotionFX.Editor, assimp, SceneAPI, the Atom .Builders modules, …). It was the only test target in o3de-extras or o3de that did this.

The test environment now assembles the spawnable in memory and registers it via AzFramework::InMemorySpawnableAssetContainer, as the Multiplayer gem does. The test body and its product_asset:///… URI are unchanged.

• Code/CMakeLists.txt — drop ${PROJECT_NAME}.Assets, add the missing Gem::ROS2
• delete Assets/SampleAsset/TestSimulationEntity.prefab — a test fixture that shipped to every project enabling the gem
• simulation-gems-pr.yml — drop Ros2GemsTest.Assets from the build targets

How was this PR tested?

Tests built and ran locally.

[jhanca-robotecai]

The tests have passed, we can draw conclusions:

  ┌──────────────────────┬──────────────────────┬────────────────┬────────────────────┐
  │                      │ baseline 35629715056 │ PR 35734424880 │         Δ          │
  ├──────────────────────┼──────────────────────┼────────────────┼────────────────────┤
  │ Build step           │ 3h 22m 34s           │ 1h 29m 37s     │ −1h 52m 57s (−56%) │
  ├──────────────────────┼──────────────────────┼────────────────┼────────────────────┤
  │ Whole job            │ 3h 33m 37s           │ 1h 39m 27s     │ −54%               │
  ├──────────────────────┼──────────────────────┼────────────────┼────────────────────┤
  │ ninja edges          │ 5750                 │ 3523           │ −2227              │
  ├──────────────────────┼──────────────────────┼────────────────┼────────────────────┤
  │ CXX objects          │ 4898                 │ 3061           │ −1837              │
  ├──────────────────────┼──────────────────────┼────────────────┼────────────────────┤
  │ Asset Processor runs │ 1 (5m 22s)           │ 0              │ —                  │
  └──────────────────────┴──────────────────────┴────────────────┴────────────────────┘