Add a test module to test after-startup shmem allocations

Add a test module to test after-startup shmem allocations

The old ShmemInit{Struct/Hash}() functions could be used after
postmaster statup, as long as the allocation is small enough to fit in
spare shmem reserved at startup. I believe some extensions do that,
although we hadn't really documented it and had not coverage for it.
The new test module covers that after-startup usage with the new
ShmemRequestStruct() functions.

Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/CAExHW5vM1bneLYfg0wGeAa=52UiJ3z4vKd3AJ72X8Fw6k3KKrg@mail.gmail.com