Test `Schema::getTableUniques()`

[Tigrov]

Schema::getTableUniques() returns only constraints of uniques but not unique indexes.

https://github.com/yiisoft/db-pgsql/actions/runs/5908951885/job/16028983991?pr=312

There was 1 failure:

1) Yiisoft\Db\Pgsql\Tests\SchemaTest::testTableUniques
Failed asserting that actual size 2 matches expected size 3.

/home/runner/work/db-pgsql/db-pgsql/tests/SchemaTest.php:570

Q	A
Is bugfix?	✔️/❌
New feature?	❌
Breaks BC?	✔️/❌
Fixed issues	#742

[what-the-diff[bot]]

PR Summary

• New Test Method Addition A new method for testing, named testTableUniques, has been added to the SchemaTest.php file. The purpose of this test method is to fetch unique constraints from a table and verify their column names.

• New Table Incorporated A new table named table_unique_index has been successfully added to the pgsql.sql file. This table contains multiple unique constraints defined within it.