Don't try to build linux-specific tests on non-linux

Some tests use linux/input.h (and epoll), but we're building on some
other kernels (e.g. debian freebsd). We could just copy the file but
it's GPL. We could also skip the tests (exit code 77) but it doesn't
really matter.

Signed-off-by: Ran Benita <ran234@gmail.com>