check 0.13.0 introduced a new struct type TTest for test functions
instead of just a function. However, now the tcase_add_* functions use
`const Ttest *`, and since litest stores the test case in a `void *`,
we get warnings like the following:

../test/test-touchpad.c:7079:30: warning: passing argument 3 of '_litest_add' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  litest_add("touchpad:fuzz", touchpad_fuzz, LITEST_TOUCHPAD, LITEST_ANY);

To fix this, use `const void *`, which is compatible with both APIs.

Signed-off-by: Michael Forney <mforney@mforney.org>