Add test for reduce with one item

[quietfanatic]

[15:17] <quietfanatic> p6: say (reduce *+*, 1, 2).WHAT
[15:17] <+camelia> rakudo-moar af886d: OUTPUT«(Int)␤»
[15:17] <quietfanatic> p6: say (reduce *+*, 1).WHAT
[15:17] <+camelia> rakudo-moar af886d: OUTPUT«(List)␤»
[15:17] <quietfanatic> Hah.  If that isn't a reportable bug, I don't know what is.

I expect reduce with one item to return that item, but it wraps it in a List instead.

This should test for that.
is-deeply(reduce(+, 72), 72, "reduce with one item returns that item")