tests: remove xcasprintf()
tests: remove xcasprintf()

There's only one user and we already removed xasprintf() which was no
longer used after introducing udev polling. Remove it for simplicity.

Signed-off-by: Bartosz Golaszewski <108de5fbc428215dc26df4468b48691526611148@gmail.com>

brgl/libgpiod,brgl/libgpiod,brgl/libgpiod,brgl/libgpiod