test_slru: Fix LWLock tranche allocation in EXEC_BACKEND builds.

test_slru: Fix LWLock tranche allocation in EXEC_BACKEND builds.

Currently, test_slru's shmem_startup_hook unconditionally generates
new LWLock tranche IDs.  This is fine on non-EXEC_BACKEND builds,
where only the postmaster executes this hook, but on EXEC_BACKEND
builds, every backend executes it, too.  To fix, only generate the
tranche IDs in the postmaster process by checking the
IsUnderPostmaster variable.

This is arguably a bug fix and could be back-patched, but since the
damage is limited to some extra unused tranche IDs in a test
module, I'm not going to bother.

Reported-by: Sami Imseih <samimseih@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0vaAuonaf12CeDddQJu5xKL%2B6xVyS%2B_q1%2BcH%3D33JXV82w%40mail.gmail.com