can't use gc(full=TRUE) on ancient R (#6546)

Author: MichaelChirico

inst/tests/tests.Rraw

@@ -19226,7 +19226,8 @@ test(2286,
        x<2, structure(list(1), class = "foo"),
        x<3, structure(list(2), class = "foo"),
        # Force gc() and some allocations which have a good chance at landing in the region that was earlier left unprotected
-       { gc(full = TRUE); replicate(10, FALSE); x<4 },
+       # TODO(R>=3.5.0): no need to branch here for gc(full=TRUE)
+       { if ("full" %in% names(formals(gc))) gc(full = TRUE) else gc(); replicate(10, FALSE); x<4 },
        `attr<-`(list(3), "class", "foo")),
      structure(list(1, 2, 3), class = "foo"))