tests: suppress "built under newer R" warning

When using the .0 release of R with binary packages from CRAN, library()
will complain about the binary package having been built under a newer
(but still compatible in this case) version of R. This was already
suppressed in most places, but not in test 2265.

Author: aitap

inst/tests/tests.Rraw

@@ -18419,7 +18419,7 @@ test(2264.8, print(DT, show.indices=TRUE), output=ans)
 if (test_bit64) local({
   DT = data.table(a = 'abc', b = as.integer64(1))
   suppressPackageStartupMessages(unloadNamespace("bit64"))
-  on.exit(suppressPackageStartupMessages(library(bit64, pos="package:base")))
+  on.exit(suppressWarnings(suppressPackageStartupMessages(library(bit64, pos="package:base"))))
   test(2265, DT, output="abc\\s*1$")
 })