test/crimson: do not discard returned future<>

test/crimson: do not discard returned future<>

to silence following warning:

../src/test/crimson/seastore/test_randomblock_manager.cc:122:7: warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]
      rbm_manager->add_free_extent(t.allocated_blocks, r.first * DEFAULT_BLOCK_SIZE,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>