Instead of having this ifdef'd out split the main and directly
associated functions out into a separate file.
That ifdef used to exist so we can use parts of litest in some other
files (the selftest and the utils test). Those tests care mostly
about the assertion helpers so long-term a split into
assert helpers and "rest of litest" would be better. For now, this will
do.
Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1174>
