tests: net: all: Increase NET_MAX_CONTEXTS from 4 to 5

DNS_RESOLVER_MAX_SERVERS has the range [1,NET_MAX_CONTEXTS], with
NET_MAX_CONTEXTS being set to 4, but is set to 5. Out-of-range .config
values are ignored, so the symbol reverts to its default value (1).

Increase NET_MAX_CONTEXTS to 5. DNS_SERVER{1-5} being set probably
implies that DNS_RESOLVER_MAX_SERVERS shouldn't be decreased to 4.

This fixes two Kconfig warnings (out-of-range + didn't get assigned
value).

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>