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.
diff --git a/programs/test/CMakeLists.txt b/programs/test/CMakeLists.txt
index 0ed7145..1e87fca 100644
--- a/programs/test/CMakeLists.txt
+++ b/programs/test/CMakeLists.txt
@@ -22,6 +22,9 @@
 add_executable(udp_proxy udp_proxy.c)
 target_link_libraries(udp_proxy ${libs})
 
-install(TARGETS selftest benchmark ssl_cert_test udp_proxy
+add_executable(zeroize zeroize.c)
+target_link_libraries(zeroize ${libs})
+
+install(TARGETS selftest benchmark ssl_cert_test udp_proxy zeroize
         DESTINATION "bin"
         PERMISSIONS OWNER_READ OWNER_WRITE OWNER_EXECUTE GROUP_READ GROUP_EXECUTE WORLD_READ WORLD_EXECUTE)