Clean-up: tests: utils: const_cast

clang-tidy warns that const_cast should not be used because of the
cppcoreguidelines-pro-type-const-cast rule.

In this case, const_cast is not strictly necessary and can be replaced
by a static_cast to silence the warning.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Iee089716e991c52686a0fdee5a7ac2aae0a30b46