pgsql: postgres_fdw: Add regression test for postgres_fdw.application_n

postgres_fdw: Add regression test for postgres_fdw.application_name GUC.

Commit 449ab63505 added postgres_fdw.application_name GUC that specifies
a value for application_name configuration parameter used when postgres_fdw
establishes a connection to a foreign server. Also commit 6e0cb3dec1
allowed it to include escape sequences. Both commits added the regression
tests for the GUC, but those tests were reverted by commits 98dbef90eb and
5e64ad3697 because they were unstable and caused some buildfarm members
to report the failure.

This is the third try to add the regression test for
postgres_fdw.application_name GUC.

One of issues to make the test unstable was to have used
postgres_fdw_disconnect_all() to close the existing remote connections.
The test expected that the remote connection and its corresponding backend
at the remote server disappeared just after postgres_fdw_disconnect_all()
was executed, but it could take a bit time for them to disappear.
To make sure that they exit, this commit makes the test use
pg_terminate_backend() with the timeout at the remote server, instead.
If the timeout is set to greater than zero, this function waits until
they are actually terminated (or until the given time has passed).

Another issue was that the test didn't take into consideration the case
where postgres_fdw.application_name containing some escape sequences was
converted to the string larger than NAMEDATALEN. In this case it was
truncated to less than NAMEDATALEN when it's passed to the remote server,
but the test expected wrongly that full string of application_name was
always viewable. This commit changes the test so that it can handle that case.

Author: Fujii Masao
Reviewed-by: Masahiko Sawada, Hayato Kuroda, Kyotaro Horiguchi
Discussion: https://postgr.es/m/3220909.1631054766@sss.pgh.pa.us
Discussion: https://postgr.es/m/20211224.180006.2247635208768233073.horikyota.ntt@gmail.com
Discussion: https://postgr.es/m/e7b61420-a97b-8246-77c4-a0d48fba5a45@oss.nttdata.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/353aa01687b5dd582debdfe63871498cd522cbbd

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out | 51 ++++++++++++++++++++++++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 35 ++++++++++++++++++
2 files changed, 86 insertions(+)