selftest/timerns: fix clang build failures for abs() calls

When building with clang, via:

    make LLVM=1 -C tools/testing/selftests

...clang warns about mismatches between the expected and required
integer length being supplied to abs(3).

Fix this by using the correct variant of abs(3): labs(3) or llabs(3), in
these cases.

Reviewed-by: Dmitry Safonov <dima@arista.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Andrei Vagin <avagin@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>