testsuite: Wrap syscall() to get calls to finit_module()

testsuite: Wrap syscall() to get calls to finit_module()

When we don't have finit_module() in libc (most likely because as of
today glibc didn't add it yet), we end up using
syscall(__NR_finit_module, ...). In this case we would not wrap the
function in the testsuite and thus having some tests failing:

TESTSUITE: ERR: could not insert module: Operation not permitted

This implementation relies on the fact that this is the only caller of
syscall(2), because we can't call libc's syscall(). There's an abort()
in place to be future safe: as soon as we need more calls to syscall(),
we can detect (and decide what to do).

Now we have all tests passing in the testsuite again.