Allow Postgres tests to be run on a different database #501

Job	Run time
Cargo fmt and clippy	12s
set-rust-versions	53s
cargo-audit	14s
Build docs	1m 12s
Test tokio-postgres (stable)	58s
Test tokio-postgres (nightly)	1m 6s
test-macros-and-cli (stable, windows-latest)	3m 27s
Test tokio-postgres (1.75)	1m 53s
Test mysql (stable)	2m 3s
test-macros-and-cli (stable, ubuntu-latest)	2m 2s
Test mysql-async (stable)	1m 12s
Test mysql (nightly)	2m 8s
test-macros-and-cli (nightly, windows-latest)	4m 15s
Test mysql (1.75)	2m 10s
Test mysql-async (nightly)	1m 7s
test-macros-and-cli (nightly, ubuntu-latest)	1m 59s
Test postgres (stable)	1m 40s
Test mysql-async (1.75)	1m 10s
test-macros-and-cli (1.75, windows-latest)	4m 9s
Test postgres (nightly)	1m 40s
test-macros-and-cli (1.75, ubuntu-latest)	2m 13s
Test postgres (1.75)	1m 40s
Test Sqlite (stable)	1m 47s
Test Sqlite (nightly)	1m 43s
Test tiberius (stable)	1m 54s
Test Sqlite (1.75)	2m 9s
Test tiberius (nightly)	1m 51s
Test tiberius (1.75)	2m 7s
CI is green	0s
	50m 54s

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