Invert boolean for migrate error message. (#3275)

[nk9]

fixes #3275

[nk9]

Question: instead of "see docs," can/should we link to the exact section that's relevant? In this case https://docs.rs/sqlx/latest/sqlx/attr.test.html#automatic-test-database-management-requires-migrate-feature

[nk9]

I see that other places in the code use e.g. cfg!(not(feature = "chrono")). I'm not sure if there's a difference between these two styles, but I am happy to align this if desired.

[abonander]

Question: instead of "see docs," can/should we link to the exact section that's relevant? In this case https://docs.rs/sqlx/latest/sqlx/attr.test.html#automatic-test-database-management-requires-migrate-feature

The biggest issue is the doc header getting changed and it not getting updated here; at most I'd link to the #[sqlx::test] attribute itself. Either way, the user should probably read the whole page to understand what they're doing anyway.

I see that other places in the code use e.g. cfg!(not(feature = "chrono")). I'm not sure if there's a difference between these two styles, but I am happy to align this if desired.

I don't care that much, but that does seem more readable.

[abonander]

@nk9 if you rebase it should fix the CI failures.