net: improved test for IPv6 pton() and ntop()

Represent IPv6 addresses as groups of 16-bit values in test vectors.

This commit corrects the following two test issues on big endian
systems:

  START - test_ipv6_pton_1
  Failed to verify ff08::
  against 0:ff08::

  START - test_ipv6_ntop_1
  Failed to verify 0:ff08::
  against ff08::

Makes the following tests pass on qemu_leon3:
- net.util

Signed-off-by: Martin Åberg <martin.aberg@gaisler.com>