Allow Postgres tests to be run on a different database #505

Job	Run time
cargo-audit	9s
set-rust-versions	4s
Cargo fmt and clippy	58s
Build docs	49s
Test mysql-async (stable)	1m 4s
test-macros-and-cli (stable, windows-latest)	4m 10s
Test mysql (stable)	2m 16s
Test mysql-async (nightly)	1m 40s
Test postgres (stable)	1m 34s
test-macros-and-cli (stable, ubuntu-latest)	1m 50s
Test mysql (nightly)	2m 6s
Test postgres (nightly)	1m 42s
Test mysql-async (1.75)	1m 30s
test-macros-and-cli (nightly, windows-latest)	4m 24s
Test mysql (1.75)	2m 6s
Test postgres (1.75)	1m 37s
test-macros-and-cli (nightly, ubuntu-latest)	1m 55s
test-macros-and-cli (1.75, windows-latest)	4m 32s
Test Sqlite (stable)	1m 38s
test-macros-and-cli (1.75, ubuntu-latest)	2m 2s
Test tiberius (stable)	1m 46s
Test Sqlite (nightly)	1m 41s
Test tokio-postgres (stable)	52s
Test tiberius (nightly)	1m 52s
Test Sqlite (1.75)	1m 39s
Test tokio-postgres (nightly)	55s
Test tiberius (1.75)	1m 58s
Test tokio-postgres (1.75)	1m 4s
CI is green	1s
	49m 54s

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