test/crimson/seastore: use gtest assertion macros instead of assert()

Plain assert() is compiled out under NDEBUG, leaving the checked
variables unused. Use the always-evaluated gtest macros instead.

  src/test/crimson/seastore/test_cbjournal.cc:586: variable 'old_written_to' set but not used [-Wunused-but-set-variable]
  src/test/crimson/seastore/test_btree_lba_manager.cc:345: unused structured binding declaration [-Wunused-variable]

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>