Modify nsapi_dns tests to be module tests

[michalpasztamobica]

Summary of changes

The nsapi_dns tests were cross-class tests anyway, going through nsapi_dns and UDPSocket. Now they also include EthernetInterface and only mock the NetworkStack, which makes them the most cross-class module test we could think of in netsocket module.

Documentation

None

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[x] Tests / results supplied as part of this PR

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Reviewers

@AnttiKauppila

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[ciarmcom]

@michalpasztamobica, thank you for your changes.
@AnttiKauppila @ARMmbed/mbed-os-ipcore @ARMmbed/mbed-os-maintainers please review.

[michalpasztamobica]

Force-pushed rebase+conflict resolution.
@AnttiKauppila , could you review, please?

[adbridge]

This still needs a maintainer review but I will put it into CI for now.

[adbridge]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts