Provide public API to call the code generated by #[sqlx::test] without involving the #[sqlx::test] itself

[vittorius]

Is your feature request related to a problem? Please describe.
It's not always convenient that #[sqlx::test] invokes the underlying async runtime. And it's a blocker for me having the test function marked with #[test]. It's because when I use harness = false for my custom test suite setup, #[test] functions are skipped by rustc and not included in the test binary.

Describe the solution you'd like
I'd like to:

1. Have the concerns of "test DB setup + run migrations" and "async runtime start" separated and opt-in for #[sqlx::test]
2. Have the option to avoid marking the test function with #[test] (when I only need the "test DB setup + run migrations" concern)
3. Maybe even have a public API that will allow calling this "test DB setup + run migrations" code explicitly, exposing the function generated by #[sqlx::test]

Describe alternatives you've considered
https://github.com/tyrchen/sqlx-db-tester

Additional context
In general, I'm trying to use #[sqlx::test] with a BDD library https://github.com/rust-rspec/rspec. And the current API of #[sqlx::test] doesn't allow me to build a custom test suite with harness = false. My idea was to have a sync top-level function marked with #[tokio::test] to be the "async main", then each test closure will manually invoke the test function generated by #[sqlx::test]

[titaniumtraveler]

Yeah, I would like that too.
Though in my case it is mostly because I would like to use #[sqlx::test] in doc tests.
(Or having the option to call it manually as you would with tokio::runtime::Builder instead of #[tokio::main])
What the #[test] attribute prevents, as functions marked as #[test] are only compiled in with the test profile, not the doctest profile

Though having looked at sqlx-core::testing, I assume that would require a fair bit of refatoring, because currently every TestFn::run_test() call starts its own runtime.