tests: avoid invalid UTF-8 warning

Under options(encoding="UTF-8"), writeLines() always tries to perform
text conversion. Giving it bytes that aren't valid in the current
encoding results in warnings. Avoid the warning by explicitly opening a
file connection with encoding="" (no conversion).

Author: aitap

inst/tests/tests.Rraw

@@ -21555,7 +21555,9 @@ test(2333, as.expression(data.table(a = 1))[["a"]], 1)
 
 # regression test for hexdigits subscript overrun (uint8_t wraps over 255, unsigned overflow is well defined in c)
 f = tempfile()
-writeLines(c('a', rep('0x1.ffffp0', 10000L), '0x1.ff\x9fp0', rep('0x1.ffffp0', 20000L)), f)
+ff = file(f, encoding = "")
+writeLines(c('a', rep('0x1.ffffp0', 10000L), `Encoding<-`('0x1.ff\x9fp0', 'bytes'), rep('0x1.ffffp0', 20000L)), ff)
+close(ff)
 test(2334, names(fread(f)), "a")
 unlink(f)