Intent of returned_map_is_a_copy

All 1 exact test in this concept. Every test is marked by where it enters the hierarchy.

go.temporal.io/ 1 test
- server/ 1 test
  - common/ 1 test
    
    contextutil/ 1 test
    
    TestContextMetadataGetAll/ 1 test
    
    go.temporal.io/server/common/contextutil/TestContextMetadataGetAll/returned_map_is_a_copy Introduced here