pgsql: Fix bogus list-iteration code in pg_regress.c, affecting ecpg te

Fix bogus list-iteration code in pg_regress.c, affecting ecpg tests only.

While looking at a recent buildfarm failure in the ecpg tests, I wondered
why the pg_regress output claimed the stderr part of the test failed, when
the regression diffs were clearly for the stdout part. Looking into it,
the reason is that pg_regress.c's logic for iterating over three parallel
lists is wrong, and has been wrong since it was written: it advances the
"tag" pointer at a different place in the loop than the other two pointers.
Fix that.

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d86e9e21058cfd53f0b6a3cabee68446aa255775

Modified Files
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src/test/regress/pg_regress.c | 14 ++++----------
1 file changed, 4 insertions(+), 10 deletions(-)