Remove dependency on DNS for Test-Connection tests on macOS

[rjmholt]

PR Summary

Fixes #12935.

macOS has had issues with DNS in CI, so this PR falls back to using the gateway address when DNS doesn't work.

Using the gateway address doesn't seem to work well on other platforms though, so we still try DNS lookup.

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[rjmholt]

From CI, it seems that solving the setup issue kicks the can down to a test failure instead where Test-Connection falls over

[rjmholt]

From the CI, it looks like ifconfig is able to resolve the address properly

[vexx32]

@rjmholt the cmdlet uses the system.net.dns APIs internally quite a bit, so I'd imagine if those APIs themselves are failing it'll most likely fall apart in at least a couple of cases.

Definitely seems like something changed that the .NET Core folks will need to take a look at... 😕

[SteveL-MSFT]

Would it be better to use -Because with the first Should test to have this in the test log?

[rjmholt]

Ok the latest change proves that this is a CI configuration issue rather than a .NET issue

[rjmholt]

Reopening to run something in CI

[adityapatwardhan]

@SteveL-MSFT Please update your review

[SteveL-MSFT]

Not blocking, but could have used the null conditional assignment operator here

[rjmholt]

This needs to port back to 6.2 -- deliberately removed new operators in most recent commit

said review was not blocking

[ghost]

🎉v7.1.0-preview.4 has been released which incorporates this pull request.:tada:

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[ghost]

🎉v7.0.3 has been released which incorporates this pull request.:tada:

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