Disable network tests; add --test-all flag to enable

[starkos]

What does this PR do?

Hides the automated tests that require calls out to the network behind a new --test-all flag. Allows for faster TDD iterations for new development, and tests can now be run even when offline.

How does this PR change Premake's behavior?

Tests which require network access (HTTP, NuGet) will no longer run by default.

Anything else we should know?

I added the --test-all flag to the GitHub CI action, so we'll still be running them there. Open to discussion on whether that's a good idea or not.

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits

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