zeroize: add #[ignore] to flaky CString test (#776)

This test relies on UB and is flaking out in certain cases:

#774

It's unclear if the test failure indicates a genuine bug in the
implementation, or is the result of a test with UB.

However, in the meantime it shouldn't block users who would just like
the test suite to run reliably.

zeroize/tests/zeroize.rs

@@ -166,7 +166,9 @@ fn zeroize_string_entire_capacity() {
  assert!(as_vec.iter().all(|byte| *byte == 0));
 }
 
+// TODO(tarcieri): debug flaky test (with potential UB?) See: RustCrypto/utils#774
 #[cfg(feature = "std")]
+#[ignore]
 #[test]
 fn zeroize_c_string() {
  let mut cstring = CString::new("Hello, world!").expect("CString::new failed");