[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

Walter Bright <bugzilla at digitalmars.com> changed:

           What    |Removed                     |Added
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                 CC|                            |bugzilla at digitalmars.com

--- Comment #4 from Walter Bright <bugzilla at digitalmars.com> ---
unittest has nothing to do with the issue. Calling it `void test()` instead
does the same thing.

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