pgsql: Add isolation test for errors during logical slot creation.

Add isolation test for errors during logical slot creation.

I didn't include this test in 2f6501fa3c5, because I was not sure the error
messages for the terminated connection is stable across platforms. But it
sounds like it is, and if not, we'd want to do something about the instability
anyway...

Discussion: https://postgr.es/m/CAD21AoDAeEpAbZEyYJsPZJUmSPaRicVSBObaL7sPaofnKz+9zg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/278cdea6b9d2993d1a819fbb28262236a72f6d02

Modified Files
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contrib/test_decoding/Makefile                     |   2 +-
.../test_decoding/expected/slot_creation_error.out | 113 +++++++++++++++++++++
.../test_decoding/specs/slot_creation_error.spec   |  41 ++++++++
3 files changed, 155 insertions(+), 1 deletion(-)