Fix tests for MySQL 8.4: add unique index for complex fks #1564

Job	Run time
code-style-and-static-analysis	2m 51s
Test Suite (7.4, sqlite, 5-min)	57s
Test Suite (7.4, sqlite, 5-max)	1m 19s
Test Suite (7.4, mysql, 5-min)	1m 11s
Test Suite (7.4, mysql, 5-max)	1m 36s
Test Suite (7.4, pgsql, 5-min)	1m 12s
Test Suite (7.4, pgsql, 5-max)	1m 38s
Test Suite (7.4, agnostic, 5-min)	50s
Test Suite (7.4, agnostic, 5-max)	1m 2s
Test Suite (8.2, sqlite, 5-min)	46s
Test Suite (8.2, sqlite, 5-max)	55s
Test Suite (8.2, sqlite, 6-min)	47s
Test Suite (8.2, sqlite, 6-max)	48s
Test Suite (8.2, sqlite, 7-min)	47s
Test Suite (8.2, sqlite, 7-max)	48s
Test Suite (8.2, mysql, 5-min)	1m 28s
Test Suite (8.2, mysql, 5-max)	1m 25s
Test Suite (8.2, mysql, 6-min)	1m 30s
Test Suite (8.2, mysql, 6-max)	1m 35s
Test Suite (8.2, mysql, 7-min)	1m 28s
Test Suite (8.2, mysql, 7-max)	1m 29s
Test Suite (8.2, pgsql, 5-min)	1m 12s
Test Suite (8.2, pgsql, 5-max)	1m 19s
Test Suite (8.2, pgsql, 6-min)	1m 9s
Test Suite (8.2, pgsql, 6-max)	1m 12s
Test Suite (8.2, pgsql, 7-min)	1m 9s
Test Suite (8.2, pgsql, 7-max)	1m 12s
Test Suite (8.2, agnostic, 5-min)	24s
Test Suite (8.2, agnostic, 5-max)	31s
Test Suite (8.2, agnostic, 6-min)	30s
Test Suite (8.2, agnostic, 6-max)	24s
Test Suite (8.2, agnostic, 7-min)	24s
Test Suite (8.2, agnostic, 7-max)	27s
	36m 15s

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