test/librados/split_op_cxx.cc: skip tests for crimson

This commit skips running the following tests with crimson: BigRead,
ReadTwoShards, StatBeforeRead, and GetXattrBeforeRead.

This is done as crimson does not support split reads.

Fixes: https://tracker.ceph.com/issues/79643
Signed-off-by: Shraddha Agrawal <shraddha.agrawal000@gmail.com>