Rework how migration_cli_test tests the preview server.
Rework how migration_cli_test tests the preview server.

This mechanism is simpler, and it has the advantage that if the
preview server fails to start for some reason, we get a useful failure
message rather than a timeout.

Change-Id: I6ead899b3af02431c2a022fa104c6596ff952917
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/145142
Reviewed-by: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk