Fix flaky testCreateTableIndexWithWhere position lookup

[dereuromark]

Summary

The test was hardcoding $queries[2] to pluck the CREATE UNIQUE INDEX statement out of the dry-run message buffer. The buffer now contains a handful of phinxlog setup messages before the create-index call:

[1] DROP TABLE "phinxlog";
[2] ALTER TABLE "tmp_phinxlog" RENAME TO "phinxlog";
[3] INSERT INTO "tmp_phinxlog"() SELECT  FROM "phinxlog";
[4] DROP TABLE "phinxlog";
[5] ALTER TABLE "tmp_phinxlog" RENAME TO "phinxlog";
[6] CREATE TABLE "table1" (...);
[7] CREATE UNIQUE INDEX "table1_email_index" ON "table1" (...) WHERE is_verified = true;

So the test was failing locally with the assertion message:

Failed asserting that 'ALTER TABLE "tmp_phinxlog" RENAME TO "phinxlog";' contains "CREATE UNIQUE INDEX "table1_email_index"".

Found while reducing the PHPStan baseline in #1083; the failure is pre-existing and reproducible on master.

Fix

Walk the message buffer and pick the matching query rather than indexing by position. The two assertions on the index DDL contents are unchanged.