pgsql: Update oidjoins regression test for v13.

Update oidjoins regression test for v13.

We seem to have forgotten to do this in the v12 cycle, so add it as
a task in the RELEASE_CHANGES list, in hopes we won't forget again.

While here, fix findoidjoins.c so that it actually works in the
new dispensation where OID is a regular column, and change it to only
consider system relations (this avoids being fooled by the OID column
in the brintest test table).

Also tweak the largeobject test so that the somewhat-recently-added
manual creation of a LO with an OID in the system range doesn't
fool findoidjoins.c.  For the moment I just made that use an unused
OID, but we might have to find a more robust solution someday.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6c298881c20797ac424558b9ff68d9781e03f6c4

Modified Files
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src/test/regress/expected/oidjoins.out       | 48 ++++++++++++++++++++++++++++
src/test/regress/input/largeobject.source    |  4 +--
src/test/regress/output/largeobject.source   |  6 ++--
src/test/regress/output/largeobject_1.source |  6 ++--
src/test/regress/sql/oidjoins.sql            | 24 ++++++++++++++
src/tools/RELEASE_CHANGES                    |  2 ++
src/tools/findoidjoins/README                |  7 ++++
src/tools/findoidjoins/findoidjoins.c        | 18 +++++++----
8 files changed, 101 insertions(+), 14 deletions(-)