network stack test cases are only tested on x86

[zephyrbot]

Reported by Andrew Boie:

It appears that all of our network stack test cases are only built for x86. The network stack is not an x86 feature, it is cross-architecture, we need to be building and running these tests on all of our supported architectures. As it stands now we could have numerous bugs that only manifest themselves on non-x86 that we don't even know about.

Acceptance criteria: remove "platform_whitelist: qemu_x86" from all the testcase.yaml under tests/net

(Imported from Jira ZEP-2398)

[zephyrbot]

by Jukka Rissanen:

#864