tests: adapt unittests for ipv6_hdr module division

Makefile.dep

sys/include/net/ipv6/hdr.h

sys/include/net/ng_ipv6/hdr.h

sys/net/link_layer/ng_netdev_eth/ng_netdev_eth.c

sys/net/network_layer/ng_icmpv6/echo/ng_icmpv6_echo.c

sys/net/network_layer/ng_ndp/internal/ng_ndp_internal.c

sys/net/network_layer/ng_ndp/node/ng_ndp_node.c

sys/shell/commands/sc_icmpv6_echo.c

tests/unittests/tests-gnrc_ipv6_hdr/Makefile

tests/unittests/tests-gnrc_ipv6_hdr/Makefile.include

tests/unittests/tests-gnrc_ipv6_hdr/tests-gnrc_ipv6_hdr.c

tests/unittests/tests-gnrc_ipv6_hdr/tests-gnrc_ipv6_hdr.h

tests/unittests/tests-ipv6_hdr/Makefile.include

tests/unittests/tests-ipv6_hdr/tests-ipv6_hdr.c