Using "-o /dev/null" fails on Windows.  Rather that working
around this in meson, add a separate command-line option so
that we can use python's os.devnull.

Reported-by: Thomas Huth <thuth@redhat.com>
Fixes: 656666dc ("tests/decode: Convert tests to meson")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230531232510.66985-1-richard.henderson@linaro.org>