Add ENV variable to skip network test

[janezpodhostnik]

Description

When making a nix derivation for flow-cli I noticed that some tests use networking. This fails in sandboxed environments. I added a SKIP_NETWORK_TESTS env variable to optionally skip them.

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Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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[jribbink]

We could also just remove them in favour of a local emulator? It was probably a mistake on my end to use networking in the first place for the fork tests.

[janezpodhostnik]

IMO they are useful, but they should perhaps be in more of an integration test suite.
We can just do this for now.