Flaky presentation test

[som-snytt]

t12308 must be a flaky test. The first warning is issued when flaking out.

    // what happens here:
    //   1. targeted type check of `foo`, warningin is suspended, then *not* reported because of the nowarn.
    //      once that type check is finished, `reportSuspendedMessages` is called
    //   2. targeted type check of `bar`, warning is directly issued because `reportSuspendedMessages` was called
    //      before in that run, for that source file; `suppressions` are considered known.
    //   3. targeted type check of `baz`, warning is directly issued, though it should be filtered out...