Add SeamendcHostTest to selinux.cpp TEST_MAPPING

SeamendcHostTest should run when making changes to selinux.cpp, to
ensure the test logic is updated to match the logic selinux.cpp follows
when it decides to compile the SELinux policy in the OpenSplitPolicy
method.

Bug: 238394904
Test: atest --test-mapping system/core/init/TEST_MAPPING:presubmit
Change-Id: Iaf0ec2356a894b071e6875dd41bcf9e842126e42