fix: ensure fresh test db's aren't accidentally deleted by do_cleanup

[phlip9]

I recently encountered a panic in our CI where a test db created by the #[sqlx::test] macro was seemingly missing, right after it was created and right before we connected to it.

thread panicked at 'failed to connect to test database: Database(PgDatabaseError { severity: Fatal, code: "3D000", message: "database \"_sqlx_test_38\" does not exist", detail: None, hint: None, position: None, where: None, schema: None, table: None, column: None, data_type: None, constraint: None, file: Some("postinit.c"), line: Some(885), routine: Some("InitPostgres") })',
    at sqlx-core/src/testing/mod.rs:243:10
stack backtrace:

   # ..
   4: sqlx_core::testing::setup_test_db::{{closure}}
             at sqlx-core/src/testing/mod.rs:240:20
   5: sqlx_core::testing::run_test::{{closure}}
             at sqlx-core/src/testing/mod.rs:216:63
   # ..

// file: sqlx-core/src/testing/mod.rs

fn run_test<DB, F, Fut>(args: TestArgs, test_fn: F) -> Fut::Output
where
    // ..
{
    crate::rt::test_block_on(async move {
        let test_context = DB::test_context(&args) // << db is created
            .await
            .expect("failed to connect to setup test database");

        setup_test_db::<DB>(&test_context.connect_opts, &args).await; // << PANIC
        // ..
    })
}

async fn setup_test_db<DB: Database>(/* .. *) {
    let mut conn = copts
        .connect()
        .await
        .expect("failed to connect to test database"); // << PANIC - db doesn't exist!
    // ..
}

Fortunately, it wasn't too hard to find the problem:

If the first test thread is a bit slow after it acquires the DO_CLEANUP permit, it can accidentally delete a fresh test db created by another thread right before that other thread has a chance to open its connection.

I have a branch (phlip9@60a5955) with some thread::sleep's sprinkled around that manages to reproduce this panic almost 100% of the time. Run the failing test with ./x.py -e postgres_15_tokio --test postgres-test-db-race or ./x.py -e mysql_8_tokio --test mysql-test-db-race.

The fix in this PR simply records the current timestamp before we acquire the DO_CLEANUP permit and only cleans up test db's created before then.

Note: the next commit in that branch contains the fix, so we can see that the issue is resolved:

phlip9@1d78ba2

[phlip9]

cc @abonander : )