pgsql: Fix tracking of temp table relation extensions as writes

Fix tracking of temp table relation extensions as writes

Karina figured out that I (Andres) confused BufferUsage.temp_blks_written with
BufferUsage.local_blks_written in fcdda1e4b5.

Tests in core PG can't easily test this, as BufferUsage is just used for
EXPLAIN (ANALYZE, BUFFERS) and pg_stat_statements. Thus this commit adds tests
for this to pg_stat_statements.

Reported-by: Karina Litskevich <litskevichkarina@gmail.com>
Author: Karina Litskevich <litskevichkarina@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CACiT8ibxXA6+0amGikbeFhm8B84XdQVo6D0Qfd1pQ1s8zpsnxQ@mail.gmail.com
Backpatch: 16-, where fcdda1e4b5 was merged

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c4758649b4db7d0c4b681985d24cc3e1e938047d

Modified Files
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contrib/pg_stat_statements/expected/dml.out | 27 +++++++++++++++++++++++++++
contrib/pg_stat_statements/sql/dml.sql      | 19 +++++++++++++++++++
src/backend/storage/buffer/localbuf.c       |  2 +-
3 files changed, 47 insertions(+), 1 deletion(-)