Merge pull request #5166 from wilzbach/dscanner-unittest-safe-or-system

Dscanner: let unittest be @safe or @system

std/exception.d

@@ -1479,7 +1479,7 @@ class ErrnoException : Exception
         super(msg ~ " (" ~ s[0 .. s.strlen].idup ~ ")", file, line);
     }
 
-    unittest
+    @system unittest
     {
         import core.stdc.errno : errno, EAGAIN;
 
@@ -1491,7 +1491,7 @@ class ErrnoException : Exception
         assert(ex.errno == EAGAIN);
     }
 
-    unittest
+    @system unittest
     {
         import core.stdc.errno : EAGAIN;
         auto ex = new ErrnoException("oh no", EAGAIN);