test/dokan: avoid runtime dependent assertion

test/dokan: avoid runtime dependent assertion

We're checking a permission denied exception message that's
runtime specific:

  /mnt/data/workspace/ceph_mingw_clang/src/test/dokan/dokan.cc:252: Failure
  Expected equality of these values:
    e.what()
      Which is: "filesystem error: in remove: Permission denied
      [\"Z:\\ro_success_b76223c4-c590-45e0-ab78-4d281ac512b5\"]"
    exception_msg.c_str()
      Which is: "filesystem error: cannot remove: No such device
      [Z:\\ro_success_b76223c4-c590-45e0-ab78-4d281ac512b5]"

In order to support libc++, we'll drop the exception message
assertion and rely on the exception type.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>