refactor(audience): mirror Runtime folder layout onto Tests

Runtime is already organised into Core/, Events/, Transport/,
Utility/. Tests has had matching subfolders since the Identity,
typed-event, JsonReader, and Log peels landed, but six fixtures
were still sitting flat at the root. Moves them under the matching
subfolder so Tests mirrors Runtime one-to-one.

Pure rename, no content change. csproj uses glob includes so no
build file update is needed.

- MessageBuilderTests.cs -> Events/
- DiskStoreTests.cs (+.meta) -> Transport/
- EventQueueTests.cs (+.meta) -> Transport/
- HttpTransportTests.cs -> Transport/
- GzipTests.cs -> Utility/
- JsonTests.cs -> Utility/

Author: ImmutableJeffrey