test: use cap_last_cap() for max supported cap number, not capability_list_length()

test: use cap_last_cap() for max supported cap number, not capability_list_length()

This test assumes capability_list_length() is an invalid cap number,
but that isn't true if the running kernel supports more caps than we were
compiled with, which results in the test failing.

Instead use cap_last_cap() + 1.

If cap_last_cap() is 63, there are no more 'invalid' cap numbers to test with,
so the invalid cap number test part is skipped.