pgsql: Try to harden insert-conflict-specconflict against autovacuum.

Try to harden insert-conflict-specconflict against autovacuum.

Looks like guaibasaurus had a autovacuum running during the
controller_print_speculative_locks step (just added in
43e08419708). Which does indeed seem quite possible.

Avoid the problem by only looking for the backends participating in
the test.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/997563dfcb2501a7a199589cd6f15f2bb8af3d04

Modified Files
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.../isolation/expected/insert-conflict-specconflict.out   | 15 ++++++++++++---
.../isolation/specs/insert-conflict-specconflict.spec     |  5 ++++-
2 files changed, 16 insertions(+), 4 deletions(-)