Move benchmark IsolateSendExitLatency to correct directory

A benchmark should be in a directory with the same name as
the benchmark main file.

Bug: #49050
Change-Id: I31c0a9f276d313ab43d61be8ac3833b77ae8ee47
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/247387
Commit-Queue: Alexander Aprelev <aam@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Auto-Submit: William Hesse <whesse@google.com>

Author: whesse