[Issue 19873] unittest should be by default @system even with -preview=dip1000

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19873

--- Comment #2 from Seb <greeenify at gmail.com> ---
I think this is what's (wrongly?) happening here which is probably why the
unittest without @system results in an error.

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