Reorganize test hierarchy

[letmaik]

The current hierarchy is organically grown and partially doesn't make sense. I suggest the following new structure which will also highlight areas that need more testing:

tests/
  core/
    abort/
    clock/
    clone/
    ...
    virtio_stdin/
  config/
    overlay/
    network/
    exit_status/
    ...
  external/
    ltp/
    iozone/
  tools/
    sgx-lkl-cfg/
    sgx-lkl-disk/
    sgx-lkl-docker/
    sgx-lkl-gdb/
  workloads/
    distros/
      alpine_3.10/
    languages/
      dotnet/
      java/
      nodejs/
      python/
    ml/
      openvino/

[letmaik]

@prp @jxyang @hukoyu Any comments on this proposal?

[prp]

@Pengpeng-Microsoft can you drive this? Thanks.

[hukoyu]

@letmaik make sense. I aggree with your recommendation.

[davidchisnall]

@Francis-Liu, please can you drive this?

[prp]

Why is this P1? This should be P2 or P3.