questionable tests

Some tests rely on things that are dubious or not mentioned in the spec. Please collect these here for further reference.

nocompile/o/offsetof_01, nocompile/offsetof_03

Expect myunion.member.offsetof and mystruct.member.offsetof to be illegal. See D bug 2358.

run/double_03

Asserts double.alignof >= 8, but the spec doesn't explicitly say that.