test: drop tests that repeat another's code path, cover the untested ones

[maxholman]

A coverage audit found tests that exercise the same code as another test and assert the same thing. Most came from running each query style through behaviour that is settled before any style-specific code runs, and from repeating what the serializer tests already show through the client. They made the suite slower to read and hid what each style actually decides. Where a case depends on a style, such as an empty array, where query-string's handling differs by array format, every style keeps its own test.

The same audit found code that no test reached: validation of async schemas and of pending values, the client's custom response validator refusing a body, error details carried over from a response body, a command's JSON and string forms, and a request body sent through the real fetcher. The body test also covers bytes backed by shared memory, which fetch would otherwise send as a comma-separated list of numbers. That test fails if the copy that prevents this is removed.