Add memory context type to pg_backend_memory_contexts view

In [1] Andres mentioned that there's no way to determine the memory
context type in pg_backend_memory_contexts. This is a bit annoying as
I'd like to add a test to exercise BumpStats().

Having the context type in the test's expected output helps ensure we
are exercising BumpStats() and any future changes to the choice of
context type in tuplesort.c gets flagged up by the test breaking.

It's probably too late for PG17, but I'll leave this here for the July CF.

David

[1] https://www.postgresql.org/message-id/20240415225749.xg7uq2hwuq2qmwpg@awork3.anarazel.de