Moves all tests into a separate package. This does not change existing
supported means of running tests (e.g. through run_tests.py).

This impacts the following APIs:
  Metadata: Key is always a str, Value is bytes for binary metadata,
    str otherwise
  Call Details: str type
  gRPC method: str type
  hostname/target: str type

Also fix spelling mistakes in the spec itself.

perf regression)

Using correct API that gives length of uncompressed data so correct amount of memory is allocated.