examples-compilation.t: always add :preamble code
examples-compilation.t: always add :preamble code

Add :preamble code to an example test even if it was marked as :solo,
fixes 2754

tbrowder/doc,tbrowder/doc,perl6/doc,MasterDuke17/doc,MasterDuke17/doc,perl6/doc,perl6/doc,perl6/doc,tbrowder/doc,MasterDuke17/doc,tbrowder/doc,MasterDuke17/doc,perl6/doc