Add programs/test/zeroize.c to test mbedtls_zeroize

The idea is to use the simple program that is expected to be modified
rarely to set a breakpoint in a specific line and check that the
function mbedtls_zeroize() does actually set the buffer to 0 and is not
optimised out by the compiler.

programs/.gitignore

@@ -47,6 +47,7 @@ test/ecp-bench
 test/selftest
 test/ssl_cert_test
 test/udp_proxy
+test/zeroize
 util/pem2der
 util/strerror
 x509/cert_app