Rather than using `cfg` to not compile expensive tests, use `cfg_attr`
to conditionally mark such tests as ignored.  In other words, instead
of writing:

    #[cfg(feature = "expensive_tests")]
    #[test]
    fn test_clear_old_data_too_many_heights() {
        // ...
    }

write:

    #[test]
    #[cfg_attr(not(feature = "expensive_tests"), ignore)]
    fn test_clear_old_data_too_many_heights() {
        // ...
    }

With this change, expensive tests will always be built which means
that i) any code changes breaking them will be caught by CI (rather
than having to wait for a nightly run) and ii) code used by expensive
tests only is no longer unused when `expensive_tests` feature is not
enabled (which means fewer `#[cfg(feature = "expensive_tests")]`
directives sprinkled throughout code).

Since we no longer mark whole modules as compiled only if the feature
is enabled, this change also means that each individual test needs to
be marked individually (rather than being able to mark whole module).
This makes it more obvious which tests are expensive and which aren’t
(since the marking is right at the test definition site) and
simplifies `check_nightly.py` script.

Issue: #4490