test: xfail some benchmarks that require external libraries or inputs

src/test/bench/shootout-k-nucleotide.rs

@@ -1,3 +1,5 @@
+// xfail-test
+
 extern mod std;
 
 use core::cast::transmute;

src/test/bench/shootout-pidigits.rs

@@ -1,3 +1,5 @@
+// xfail-test
+
 use core::cast::transmute;
 use core::from_str::FromStr;
 use core::libc::{STDOUT_FILENO, c_char, c_int, c_uint, c_void, fdopen, fputc};

src/test/bench/shootout-reverse-complement.rs

@@ -1,4 +1,5 @@
 // xfail-pretty
+// xfail-test
 
 use core::cast::transmute;
 use core::libc::{STDOUT_FILENO, c_int, fdopen, fgets, fopen, fputc, fwrite};