Allow Postgres tests to be run on a different database #493

Job	Run time
Build docs	1m 10s
Cargo fmt and clippy	1m 20s
cargo-audit	16s
set-rust-versions	8s
Test Sqlite (stable)	1m 39s
Test Sqlite (nightly)	1m 45s
Test mysql (stable)	2m 8s
Test mysql-async (stable)	1m 9s
Test Sqlite (1.75)	1m 45s
Test mysql (nightly)	2m 20s
test-macros-and-cli (stable, windows-latest)	4m 9s
Test mysql-async (nightly)	1m 11s
Test mysql (1.75)	2m 13s
test-macros-and-cli (stable, ubuntu-latest)	1m 58s
Test mysql-async (1.75)	1m 9s
test-macros-and-cli (nightly, windows-latest)	4m 51s
Test postgres (stable)	1m 40s
test-macros-and-cli (nightly, ubuntu-latest)	1m 57s
Test postgres (nightly)	1m 51s
test-macros-and-cli (1.75, windows-latest)	4m 45s
Test tiberius (stable)	1m 50s
Test tokio-postgres (stable)	55s
Test postgres (1.75)	1m 51s
Test tiberius (nightly)	1m 57s
test-macros-and-cli (1.75, ubuntu-latest)	1m 58s
Test tokio-postgres (nightly)	55s
Test tiberius (1.75)	1m 55s
Test tokio-postgres (1.75)	57s
CI is green	1s
	51m 43s

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