[YSQL] flaky test PgIndexBackfillTest.CreateIndexSimultaneously

Jira Link: DB-10724

Description

According to (internal link) https://detective.dev.yugabyte.com/stability/test?branch=master&class=PgIndexBackfillTest&name=CreateIndexSimultaneously&num_fails=300, it appears this test started failing with the following message around 10/27/2021 04:38:

../../src/yb/yql/pgwrapper/pg_index_backfill-test.cc:627
Value of: std::find_if( std::begin(allowed_msgs), std::end(allowed_msgs), [&msg] (const std::string allowed_msg) { return msg.find(allowed_msg) != std::string::npos; }) != std::end(allowed_msgs)
  Actual: false
Expected: true
Network error (yb/yql/pgwrapper/libpq_utils.cc:219): Execute of 'CREATE INDEX iii ON ttt (i)' failed: 7, message: ERROR:  Operation expired: Transaction 246ab4cf-2617-48c4-b156-a23982d92675 expired or aborted by a conflict: 40001
 (pgsql error 40001)

This issue still exists today. Possible cause is commit 888c0d6b2478cba5de906a09ffe1d96c037f488a, but I don't have much confidence in that. Fix seems to be adding "expired or aborted by conflict" to the list of allowed messages, as is done for a different test in the same file.

Issue Type

kind/failing-test

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