tests/test-arm-mptimer: Don't leak string memory

tests/test-arm-mptimer: Don't leak string memory

The test-arm-mptimer setup creates a lot of test names using
g_strdup_printf() and never frees them. This is entirely
harmless since it's one-shot test code, but it clutters
up the output from clang's LeakSanitizer. Refactor to
use a helper function so we can free the memory.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181204132952.2601-5-peter.maydell@linaro.org