80f787e ("*: replace syscall with unix") did not correctly replace
the usage of (*syscall.Stat_t).[AM]tim.Unix() in tests, which resulted
in 32-bit builds failing in OBS because we run those tests in a %check
section. We need to add CI testing to make sure that we don't miss stuff
like this in the future.

Fixes: 80f787e ("*: replace syscall with unix")
Signed-off-by: Aleksa Sarai <asarai@suse.de>

This also adds a local-validate-build target which validates that the
project can be built (including the tests), which is then run as part of
our CI.

Signed-off-by: Aleksa Sarai <asarai@suse.de>