crimson/os/seastore: initialize tests in seastar reactor

crimson/os/seastore: initialize tests in seastar reactor

testing::Test from gtest is constructed and destructed outside seastar
application. It is unexpected for most seastore components, and causing
undefined behaviors.

For example, when registering metrics during construction, the logic
needs to be called inside seastar reactor thread because metrics are
maintained in reactor thread-local variables.

Signed-off-by: Yingxin Cheng <yingxin.cheng@intel.com>