Use setattr to avoid shallow copy in autoprint test setup (#7102)

* try setattr

* Update autoprint.Rout.save

Author: MichaelChirico

tests/autoprint.R

@@ -58,7 +58,7 @@ local({
 
 # child class of data.table doesn't induce unintended print, #3029
 dt = data.table(x = 1)
-class(dt) = c("foo", "data.table", "data.frame")
+setattr(dt, "class", c("foo", "data.table", "data.frame"))
 print.foo = function(x, ...) {
   NextMethod("print")
 }

tests/autoprint.Rout.save

@@ -172,7 +172,7 @@ NULL
 > 
 > # child class of data.table doesn't induce unintended print, #3029
 > dt = data.table(x = 1)
-> class(dt) = c("foo", "data.table", "data.frame")
+> setattr(dt, "class", c("foo", "data.table", "data.frame"))
 > print.foo = function(x, ...) {
 +   NextMethod("print")
 + }